712 THE ANIMAL KINGDOM. 



secretes an external hard brown chitinoid coat. It is therefore rigid, and 

 is termed ' pectocaulus ' by Professor Ray Lankester. It becomes 

 adherent to, and eventually sinks into the substance of the tubarium on 

 its attached aspect. The mouth lies beneath the epistome. The ali- 

 mentary canal is ciliated, and has a U-shaped curvature as in Polyzoa. 

 The anus is situated on a more or less projecting papilla at the head-end 

 of the body, but on the opposite side to the mouth. There is a small 

 coelome which extends into the stalk. It is lined in Rhabdopleura by 

 fusiform cells, sometimes branched, occasionally stretching from the body- 

 wall to the digestive tract. The ectoderm cells of the arms, tentacles, 

 epistome, and to a lesser degree of the stalk, contain in Rhabdopleura 

 some an orange, others a black, pigment ; and there is a special aggre- 

 gation of black pigment at one end of the epistome, possibly a rudi- 

 mentary eye. Cephalodiscus has a pair of eyes apparently placed on the 

 oral aspect above the mouth, and hidden by the buccal shield. RJiabdo- 

 pleura has a ciliated sensory (?) tubercle at the base of each arm on its 

 aboral aspect. No nervous system and no nephridia have been detected. 

 The testis in Rhabdopleura is an elongated sac opening by a special pore 

 anteriorly and aborally. Ova have been found in Cephalodiscus. Both 

 organisms reproduce by budding. While the buds are detached in 

 Cephalodiscus, they remain connected by the pectocaulus in Rhabdopleura. 

 The young buds in this genus are formed on the soft stalk or gymnocaulus 

 of a zooid, which appears itself to remain for a time in an arrested con- 

 dition, i. e. its arms are rudimentary, its buccal shield deeply bifid anteriorly, 

 its gymnocaulus in a state of growth. The youngest bud is the one 

 nearest to it. The buds are successively isolated with increasing age 

 from one another by transverse septa which divide the axial tube into 

 compartments (supra]. 



The affinities of Pterobranchia are at present doubtful. In the absence of all 

 embryological evidence it is not possible to say whether the buccal shield or epistome 

 is prae-oral as it is in Brachiopoda and Vermiformia, or post-oral, as it appears to be 

 in the entoproctous Polyzoa ; nor again whether or no the flexure of the intestine is 

 dorsal or ventral. The class is accordingly treated as independent here. 



Ray Lankester, ' Polyzoa,' Encyclopaedia Britannica (ed. ix.), xix. p. 434. 



Rhabdopleura, Id. Q. J. M. xxiv. 1884. 



Cephalodiscus, M c lntosh, A. N. H. (5), x. 1882. 



CoELENTERATA. 



METAZOA in which a gelatinoid substance, the supporting lamina or 

 mesoglaea 1 ) intervenes between the epi- and hypo-blast of the embryo or 



1 The term 'mesoglaea' is due to Mr. G. C. Bourne (Q. J. M. xxvii. pt. 3, 1887). It seems 

 unadvisable to apply either of the now synonymous terms ' mesoblast ' and ' inesoderm ' to the sup- 



