Characteristics of Polyzoa. ci 



gated and vasiform, with one end directly connected with that side 

 of the branchial organ which is opposite to that nearest the rectum. 

 Its action is periodically reversed during life ; but this end of the 

 heart may be regarded as the homologue of the vessels which bring 

 blood to the heart, whilst the other end may be considered to 

 represent the systemic aorta of bivalves. The Tunicata are her- 

 maphrodite, but may multiply by gemmation as well as sexually. 

 With the exception of Saljm, they all go through a metamorphosis, 

 the larval form being caudate and active. 



The larvae of certain sessile Ascidians, Phallusia mammillata, Phallusia 

 intestinalis, and Phallusia canina, have been recently described by Kowa- 

 lewsky and Kupffer as possessing in their caudal appendage a structure 

 closely similar to the chorda dorsalis previously held to be a distinctive 

 characteristic of Vertebrata ; as having their nerve-centres formed by the 

 fusion of lamellar upgrowths into tubes, in the manner which had 

 been similarly supposed to be peculiar to the higher Sub-kingdom, 

 thence spoken of as ' bicavitary ; ' and finally as having the caudal axis- 

 cylinder resembling the vertebrate chorda dorsalis either actually inter- 

 posed between one part of their nerve-centres, or, at least, so placed, that 

 if prolonged, it would come to be so interposed. ... It may be added 

 that Professor Gegenbaur, in the recently published second edition of his 

 'Grundziige der Vergleicheuden Anatomie,' p. 158, 1870, has placed the 

 Tunicata in the Sub-kingdom Vermes, assigning in justification of this 

 step the fact that the peculiar specialization of the anterior segment of 

 their digestive tract as a respiratory organ finds a parallel in the organ- 

 ization of a rai'e order (or Class) of worms, the Enteropieusti, represented 

 by two species, the Balanoglossus clavigerus, and the Balanoglossus 

 Tninutus found on the Neapolitan coast. See Kowalewsky, Mem. Acad. 

 Imp. St. Petersburgh, Ser. vii., Tom. x. 3, 1866. Against this is to be 

 set the close correspondence in the way of homologies Avhich may be 

 shewn (see infra, pp. 66-69, 235, 236) to exist between at least the 

 adult Tunicate and the Lamellibranchiate organism. 



Class, Polyzoa. 



Molluscoidea which are always social; and, with the excep- 

 tion of Cristatella and, perhaps, also Lophofus and Sclenariadae, 

 always fixed in their adult state, in a ^polyzoary' or '^ coenoe- 

 cium.' This structure, which is cither erect or adnate, and, under 



