INVERTEBRATE ZOOLOGY. 43 



The present Cephalods are the few degenerate descendants of a very large 

 and abundant group, which filled the seas in Palaeozoic and Mesozoic times. 

 They possessed, originall}', well developed shells, divided into chambers ; some 

 shells being straight, others spirally coiled. The orders of Nautiloidea and 

 Ammonoidea were Tetrabranchs, the Belemnoidea Dibranchs. 



Cladus III. Molluscoidea. 



Class I. Bryozua (Polyzoa) Miniite forms, usually colonial. At an- 

 terior end a ridge, the lophophore, which 

 bears ciliated tentacles. Anus situated 

 outside the lophophore. 



Order 1. Phoronidea Worm-like forms, enclosed in leathery 



tubes. Larva free-swimming — the " Ac- 

 tinotrocha." Similar to the trochophore 

 — one genus. PJioronin. 



Order 2. Ectoprocta Typical forms, nearly alwaj^s forming a 



colony, which resembles an alga. Each 

 animal is enclosed in a transparent cell, 

 from which it may extend its tentacles, 

 and into which it may entirely withdraw. 

 A few fresh water forms, the rest marine. 

 ■ifBugula. Plumatella. 



Class II. BiiACUioroDA All marine, depi'essed in form, with dor- 

 sal and ventral shells, which are sym- 

 metrical, but unequal. Mouth situated 

 between two spiral ciliated arms, which 

 lie coiled up in the shell — a large fossil 

 order. Few living. 



Order 1. Ecardines Shell without hinge, if Lingnla. 



Orders. Testicardines. . . . Shell with hinge, visually calcareous 



loops to support arms. -^ Terebratulina. 



Type V. Ambulacralia. 



Cladus. Echinodermata. 



Class I. Holothuroidea Adult creeping or sessile, oval or vermi- 

 form, covered by a leathery integument 

 in which minute calcareous spicules, 

 plates, etc.. lies imbedded, Around the 



