572 THE ANATOMY OF INVERTEBRATED ANIMALS. 



of Nematoid worms; while tbe Ectoprocta^ in possessing a 

 perivisceral cavity with a special lining, the inner surface of 

 which may be ciliated, are, so far, comparable to Brachiopods 

 or Echinoderms. 



Unfortunately, our knowledge of the embryonic develop- 

 ment of the ectoproctous Polyzoa does not enable us to de- 

 termine with certainty the nature of this perivisceral cavity, 

 and of the layer which bounds it. Nitsche shows that the 

 saccular cystid, which results from the first developmental 

 changes of the embryo in the Phylattoloimata, is composed 

 of two layers, which correspond with those of the endocyst 

 in the adult ; and, further, that the polypide (alimentary 

 canal, tentacula, and ganglion) results from an ingrowth of 

 the outer layer of the endocyst, w^hich pushes before it an in- 

 volution of the inner layer. The latter gives rise to the re- 

 flected " peritoneum." 



But I am not aware that there is any evidence which 

 proves conclusively the manner in which these two layers of 

 the embryonic endocyst take their origin, or with what layers 

 of the ordinary embryo they are homologous. If we make 

 the ordinary assumption that the inner or peritoneal layer of 

 the endoc3^st is the partial or complete homologue of the hy- 

 poblast in other animals, it follows that the perivisceral cav- 

 ity of the Ectoprocta is really an enterocoele, as it is in the 

 Brachiopoda. The oidy other alternative appears to be the 

 supposition that the inner laj^er of tlie endocyst is a meso- 

 blast, differentiated from the germ earlier than the hypoblast; 

 in which case the perivisceral cavity will be a schizocoele. 



Dr. Jhering's work on the.nervous system of the Mollusca^ 

 to which 1 have already referred, contains a number of valu- 

 able anatomical details, and especially gives a better account 

 of the structure of the nervous system of Chiton than has 

 hitherto existed.^ 



1 In addition to a great variety of surprising phylogenic speculations, Dr. 

 Jhering puts forward the novel morphological views that the respiratory sac 

 of the Pulmonata {Nephrnpnei'sta, Jhering) is morphologically a sort of urinary 

 bladder, and that the ganglia w^hence the'arm-nerves of the Cephalopoda arise 

 are cerebral, and not pedal. The arms are thus parts of the head, and only the 

 funnel represents the foot of Gasteropods. 



I do not presume to rebel against the authoritative censure of my memou- on 

 the " Morpholoiry of the Mollusca," published now five-and-twenty years ago. 

 which is pronounced by Dr. Jhering. Nevertheless, I may remark that, had 

 he condescended to pav attention to what is said respecting the flexure of the 

 intestine in Mollusks in that antiquated production, he would not have com- 

 mitted himself to the publication of the two diagrams— one of a Cephalopod 

 and the other of a Pteropod— each with its alimentary canal twisted after a 

 fashion of which Nature knows nothing, which illustrate, though they hardly 

 adorn, page 272 of his work. 



