18 Mr. H. J. Carter on the Polype-like 



avec les polypes. Quant h, moi, jc ne crois pas que ces plis 

 puissent etrc compares h. aucune partie du corps d'une polype, 

 8oit aux bras, soit aux plis in^sentdriaiix. C'est une simple 

 analogic dc forme, rien de plus." (M(3m. pub. par la Soc. des 

 Arts et des Sci. d'Utrecht, pp. 11 & 12, pi. 4. fig. 12 &c.) 



As regards homology and adaptation, it is manifest that if 

 the pores are to be considered the homologues of the ends of 

 the gastro-ventricular canals of an Actinia, which are said to 

 open on its surface, then their tentacle-like structure cannot 

 be considered homologous with the tentacles round the mouth 

 of the Actinia or polype. 



Then, as regards the function of the vent and the excretory- 

 system of canals generally, it is the rule, and not the excep- 

 tion, for the current to pass outwards, and vice versa. Indeed 

 the structural aiTangemcnt in all sponges about the vent proves 

 this ; and where the opposite takes place, it seems to me to be 

 occasioned by abnormal conditions, similar, perhaps, to what 

 Dr. Bowcrbank has stated to occur on such occasions. (" Ult. 

 Struct. Marine Sponges," Annals, Oct. 1870, vol. vi. p. 331.) 



In all sponges which are living and active, the inhalant and 

 exhalant functions of the pores and vents respectively may be 

 easily seen by placing a little colouring-matter in the water 

 which surrounds them, when the process will be found to be 

 almost invariable. 



For the development of the seed-like body of SjJongiUa and 

 the spicule, see ' Annals,' 1848, vol. i. p. 305 ; ih. 1849, vol. iv. 

 p. 82 &c. ; ih. 1857, vol. xx. p. 26 ; and ih. 1859, vol. iii. p. 334, 

 respectively, wherein I am pleased to observe that much has 

 been confirmed by Prof. ITackel's observations on the calcareous 

 sponges, to which I shall presently allude more particularly. 



Lastly, I have given an illustration of a group of Botryllus 

 poJycyclus (PI. II. fig. 41), to show how the Ascidians of 

 which it is composed have each its separate branchial aper- 

 ture (c), for aeration and nutrition, on the surface of the gela- 

 tinous mass (a) in which they are imbedded, and its anal 

 orifice {d) internally, extended into a common receptacle or 

 cloacal cavity [e), which finally also opens externally on the 

 same surface, for the discharge of the faecal contents of the 

 little community generally ( /'), there being a great many 

 communities of the same kind imbedded in the same fiat and 

 spreading, tough, gelatinous or albuminous mass. 



Now here we cannot help seeing that tlie gelatinous mass 

 is at least analogous to the sponge-structure (indeed in the 

 little white incrusting species Lvptodinum yehitinosum it is 

 also densely eharged with globular radiated calcareous bodies 

 (spicules) similar to some of the siliceous ones of the Geodidw, 



