MUSEUM OF COMPARATIVE ZOOLOGY, 91 



layer in the diflFerent Ectoproct larvte that we must look for an explana- 

 tion of the layer in the specihc case of the Phylactokemata. 



For the purposes of this study, it is desirable to begin with species in 

 which there has been a mininmm amount of degeneration. Such are 

 Merabranipora (Cyphonautes), Alcyonidium, and Flustrella, to which 

 we must now turn our attention. 



The studies of EepiachoflF on Membranipora lead up to a stage in which 

 the entoderm lies as a solid mass inside the ectoderm, and is separated 

 from it at all points. Xeither the origin of the mesoderm nor the forma- 

 tion of stomodseum or proctodseum was observed at this time. As for 

 the fully formed Cyphonautes, it is certain, as I can confirm from personal 

 observation, that there is a well developed functional alimentary tract, 

 and that it is provided with a well developed muscular system, including 

 cross-striped muscle fibres. Tliere is, therefore, every reasou for believing 

 that typical entoderm and mesoderm have been formed in it. 



In Alcyonidium (polyoum), Harmer ('87, p. 445) has shown that after 

 gastrulation a great mass of cells occupies the foruier blastocoel. This, 

 in the author's opinion, represents entoderm and mesoderm. The young 

 larva possesses a mouth, an oesophagus, and a large stomach, but never 

 an anus. No evidence is presented that the oral pole coiTespouds with 

 the pole of ingression. » 



Flustrella, which is nearly related to the last species, possesses in 

 its young larval stages a pocket, which Prouho ('90, pp. 424-426) has 

 shown to represent the anterior part of the alimentary tract, directly 

 comparable with that of Alcyonidium polyoum, but less developed. Mus 

 cle fibres and an epithelial lining of the entoderm and ectoderm exist to 

 indicate the presence of mesodermal tissue. 



These three genera, Membranipora, Alcyonidium, and Flustrella, are 

 the only Ectoprocta in whose larvae the presence of an alimentary tract 

 has as yet been demonstrated. 



In Bugula, a very careful study of which was made by Yigelius ('86 

 and '88), one finds after gastrulation and cell multiplication a mass of 

 cells filling the whole interior of the larval body, at first appearing as 

 an epithelium surrounding a central space, but later without arrangement 

 and often showing signs of degenerescence. No definite separate meso- 

 derm could be found, and at no time was any trace of an alimentary tract 

 to be seen. Vigelius calls the mass derived from the four entodermal 

 cells Fiillgewehe, and he believes it to correspond morphologically to 

 both " hypoblast and mesoblast." It is to be noted, however, as a 

 point of considerable importance, that in his figures of the metamorphos- 



