{"topic":"thyrostraca","count":1,"articles":[{"id":"britannica-11-thyrostraca","work_id":"britannica-11","title":"Thyrostraca","normalized_title":"thyrostraca","slug":"thyrostraca","aliases":[],"subtitle":"Internet Archive OCR import","body_text":"THYROSTRACA, an order of Crustacea, comprising barnacles, acorn shells and some allied degenerate parasites. The embryos are free-swimming, active forms, but in adult life the animals are fixed head downwards, and are very degenerate. The body is indistinctly segmented, and is enveloped in a fold of the integument, usually with calcareous plates. The anterior antennae are fused with the anchoring attachment, whilst the posterior pair is vestigial, and the appendages of the mouth and body present various degrees of degeneration and specialization. In most cases the adults are hermaphrodite, but unisexual forms also occur, whilst the hermaphrodite adults may carry with them minute \" complementary \" males. In strong contrast with the condition in most Crustacea, the spermatozoa are mobile. As shown by Burmeister in his historical review (1834), these animals, comprised by Linnaeus in the genus Lepas, first received a more comprehensive title from Cuvier, who called them Cirrhopoda, a word strictly meaning tawnyfooted. Lamarck in 1809 altered this into the hybrid form Cirrhipoda, meaning curl-footed, which was subsequently improved into Cirripedia or Cirrhipedia. So long as the group was held to be a subordinate member of the Entomostraca, this term, though not the earliest, was generally accepted. The name Thyrostraca, meaning doorshells or valve-shells, is preferred as agreeing in termination with the titles of the other two divisions, the Malacostraca and Entomostraca. The group may conveniently be arranged in two principal sections — the Genuina with cirrhiform feet, and the Anomala without them. Thyroslraca genuina. — It is with these that Darwin's classical treatises (Ray Soc. and Palaeont. Soc., 1851-1854) are almost exclusively concerned. Therein an order Thoracica comprehends the pedunculate Lepadidae, together with the operculate and sessile Balanidae and Verrucidae; a single species without cirrhi constitutes the order Apoda, and a single species with only three pairs of cirrhi the order Abdominalia. Within the last KoMorine (Noll, 1872) with two species, and Lithoglyptes (Aurivillius, 1892) with three species, have since been included. But H. J. Hansen (Die Cirripedien der Plankton-Expedition, 1899) states that Cryptophialus minutus, for which the order Abdominalia was founded, has, like Alcippe and other Genuina, its cirrhi on the thorax, not, as Darwin wrongly supposed, on the abdomen. In place, therefore, of the Abdominalia, it will be right to accept the family Cryptophialidae, v. Martens, side by side with the Lithoglyptidae of Aurivillius and the Alcippidae of Gerstaecker. These, with Darwin s three families above mentioned, complete the section of genuine cirripedes now existing. Gruvel submitted to the Linnaean Society a rearrangement of the Lepadidae, unfortunately using for the first of his new families the preoccupied name Anaspidae. It is confusing, but not uninstructive, to find that within the Balanid group such generic titles as Stephanolepas and Platylepas have been coined. The vernacular name barnacle, traceable to the fable of pedunculate cirripedes hatching out into bernicle geese, has also been transferred to the sessile cirripedes, which are popularly known as acorn barnacles. A complete list of all the recent species of Thyrostraca in both sections, down to the year 1897, was published by Weltner (Arch. Nature., 1898, § 63. pt. L pp. 227-280). For fossil species, Woodward^ Catalogue of Brit. Foss. Crust. (1877), pp. 137-144, should be consulted. Hoek •906","body_html":"

THYROSTRACA, an order of Crustacea, comprising barnacles, acorn shells and some allied degenerate parasites. The embryos are free-swimming, active forms, but in adult life the animals are fixed head downwards, and are very degenerate. The body is indistinctly segmented, and is enveloped in a fold of the integument, usually with calcareous plates. The anterior antennae are fused with the anchoring attachment, whilst the posterior pair is vestigial, and the appendages of the mouth and body present various degrees of degeneration and specialization. In most cases the adults are hermaphrodite, but unisexual forms also occur, whilst the hermaphrodite adults may carry with them minute " complementary " males. In strong contrast with the condition in most Crustacea, the spermatozoa are mobile. As shown by Burmeister in his historical review (1834), these animals, comprised by Linnaeus in the genus Lepas, first received a more comprehensive title from Cuvier, who called them Cirrhopoda, a word strictly meaning tawnyfooted. Lamarck in 1809 altered this into the hybrid form Cirrhipoda, meaning curl-footed, which was subsequently improved into Cirripedia or Cirrhipedia.

\n

So long as the group was held to be a subordinate member of the Entomostraca, this term, though not the earliest, was generally accepted. The name Thyrostraca, meaning doorshells or valve-shells, is preferred as agreeing in termination with the titles of the other two divisions, the Malacostraca and Entomostraca. The group may conveniently be arranged in two principal sections — the Genuina with cirrhiform feet, and the Anomala without them. Thyroslraca genuina. — It is with these that Darwin's classical treatises (Ray Soc. and Palaeont. Soc., 1851-1854) are almost exclusively concerned. Therein an order Thoracica comprehends the pedunculate Lepadidae, together with the operculate and sessile Balanidae and Verrucidae; a single species without cirrhi constitutes the order Apoda, and a single species with only three pairs of cirrhi the order Abdominalia. Within the last KoMorine (Noll, 1872) with two species, and Lithoglyptes (Aurivillius, 1892) with three species, have since been included. But H. J.

\n

Hansen (Die Cirripedien der Plankton-Expedition, 1899) states that Cryptophialus minutus, for which the order Abdominalia was founded, has, like Alcippe and other Genuina, its cirrhi on the thorax, not, as Darwin wrongly supposed, on the abdomen. In place, therefore, of the Abdominalia, it will be right to accept the family Cryptophialidae, v. Martens, side by side with the Lithoglyptidae of Aurivillius and the Alcippidae of Gerstaecker. These, with Darwin s three families above mentioned, complete the section of genuine cirripedes now existing. Gruvel submitted to the Linnaean Society a rearrangement of the Lepadidae, unfortunately using for the first of his new families the preoccupied name Anaspidae. It is confusing, but not uninstructive, to find that within the Balanid group such generic titles as Stephanolepas and Platylepas have been coined. The vernacular name barnacle, traceable to the fable of pedunculate cirripedes hatching out into bernicle geese, has also been transferred to the sessile cirripedes, which are popularly known as acorn barnacles. A complete list of all the recent species of Thyrostraca in both sections, down to the year 1897, was published by Weltner (Arch. Nature., 1898, § 63. pt. L pp. 227-280).

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For fossil species, Woodward^ Catalogue of Brit. Foss. Crust. (1877), pp. 137-144, should be consulted. Hoek •906

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