ClASSII'ICATION OJP THE ANIMAL KINGDOM. 213 



but it is possible that tbe lower forms are Enteroccelous, like the 

 members of the next division*. 



The Enteroccela. — Kowalewsky has shown that in the Chseto- 

 gnatha, represented by the strange and apparently anomalous 

 Sagitta, the vitellus undergoes complete segmentation, and is con- 

 verted into a vesicular Morula, on one side of which invagination 

 takes place, and gives rise to the primitive alimentary canal, of 

 which the opening of invagination becomes the permanent anus, 

 tbe mouth being formed, by perforation, at the opposite end of the 

 body. Before the mouth is formed, however, the primitive ali- 

 mentary cavity throws out, on each side, a csecal pouch, which ex- 

 tend as far forward as its central continuation ; while posteriorly 

 these pouches stretch behind the anus, meeting, but remaining 

 separated by their applied walls, in the median plane of the body. 

 These lateral sacs are next shut off from the median portion of the 

 primitive alimentary cavity, which becomes the permanent alimen- 

 tary canal ; and they are converted into closed sacs, the cavity of 

 each of which forms one half of the perivisceral cavity, while the 

 inner wall, applied to the hypoblast, gives rise to the muscular 

 wall of the intestine, and the outer wall, applied to the epiblast, 

 becomes the muscular wall of the body, and gives rise to the 

 generative organs. The great ganglia and nerves are developed 

 from the cells of the epiblast. "We have thus an animal which is 

 temporarily coelenterate, but in which the two gastrovascular sacs, 

 enclosiag what may be termed an " enteroccele," become shut off 

 and metamorphosed into parts of exactly the same order as those 

 which arise from the mesoblast of an Annelid. But it is not 

 altogether clear whether the cells of the enterocoele in this case 

 give rise only to the lining of the perivisceral cavity, and Avhether 

 the muscles and connective tissue are in fact derived from the 



* When I wrote this paragraph, I had been for some time in possession of 

 the recent important memoir on the development of the Brachiopoda by M. 

 Kowalewsky, as that distinguished embryologist had been good enough to send 

 it to me. But it is written in Russian, and I could only judge from the figures 

 that the periTisceral cavity of Argioj^e is developed in the same way as that of 

 Sagitta. Some little time ago, however, my friend Mr. W. F. Ralston kindly 

 took the trouble to translate so much of the text as refei-red to these figures for 

 me, and I found that my interpretation of them was correct. The Brachio- 

 poda, or some of them, therefore, are Enteroccela; and their relations with the 

 schizocoele Annelida and MoUusca bring up anew the question suggested by the 

 frequent origin of the mesoblast from the hypoblast (as in the Sharks for example), 

 May not the schizoccele be derivable from a primitive enterocoele condition? 



15* 



