CIKRIPEDIA. 423 



Ascothoracica are parasitic. The relations between the Rhizo- 

 cephala and their Crustacean hosts are among the most astonish- 

 ing examples of parasitism to be found in natural history. 



The Lepadidae are represented in Ordovician strata by 

 examples of the Pollicipedidae (including the existing genus 

 Pollicipes) and attain their culminating point during the 

 Cretaceous period. The curious unsymmetrical Verrucidae 

 appear in the Cretaceous, but the other groups of Operculata 

 are not known prior to the Tertiary period. 



Sub-order 1. CIRRIPEDIA GENUINA. 



Tribe 1. PEDUNCULATA. Body stalked, with six pairs of biramous 

 feet. Scuta, terga and a carina are usually formed on the mantle, and 

 when other plates are present they are not united into an immovable 

 ring. 



Fam. 1. Lepadidae. The stalk is sharply marked off from the capi- 

 tulum, and calcareous plates are not developed on it. The plates on the 

 capitulum are thin, their number does not usually exceed five, and the 

 terga lie behind the scuta. Hermaphrodite. Lepas L. (Figs. 272 and 273) 

 attached to floating objects ; L. anatifera L., like most of the species of 

 the genus, cosmopolitan, from arctic to tropical seas. Megalasma Hoek, 

 Poecilasma Darw., generally attached to Crustacea. Oxynaspis Darw. , 

 Dichelaspis Darw., the calcareous plates on the mantle are separated from 

 one another by wide intervals, and the scuta and terga are deeply notched ; 

 they live attached to sea-snakes or crabs. Conchoderma Olfers, capi- 

 tulum in the main membranous, the plates are small and may be reduced 

 to two (scuta). Cosmopolitan, attached to floating objects, living or in- 

 organic. Alepas Rang., capitulum without plates, or with horny almost 

 hidden scuta ; attached to various floating objects ; A. parasita Rang. 

 on medusae. Anelasma Darw., A. squalicola Loven, the peduncle is em- 

 bedded in the skin of the sharks Squalus maximus and Spinax living in 

 the North Sea, the skin of the cirripede being prod\iced into branching 

 rootlike processes, knobbed at their ends, which ramify in the tissues of 

 the fish. The capitulum is witjhout plates and has a wide aperture. The 

 six pairs of legs have a shapeless appearance ; they are obscurely arti- 

 culated and without setae. Gymnolepas Auriv., pelagic, on medusae ; 

 cirri articulated and setose. Chaetolepas Studer, on sertularians. 



Fam. 2. Pollicipedidae. The stalk usually obscurely divided from 

 the capitulum, and covered with calcareous scales or chitinous hairs. 

 Capitulum with numerous massive plates, frequently exceeding five in 

 number ; the terga are rather dorsal than posterior to the scuta. Many 

 species are hermaphrodite, some with complemental males, and some 

 are dioecious. Pollicipes Leach, stalk closely covered with scales or 

 spines ; in addition to the five plates of the Lepadidae, rostral and lateral 

 plates are strongly developed, and many smaller additional plates (18 

 to 100 or more) clothe the base of the capitulum ; hermaphrodite ; at- 

 tached to fixed or floating objects in the warmer seas of the globe. 

 P. signatus Aur., occurs in Silurian of the I. of Wisby in the Baltic. 



