246 ECTOPROCTA. 



larva atrophies, while the dorsal organ alone develops into the fixed 

 form 1 . 



Although the changes which take place during budding do not fall 

 within the province of this work, it may be well to state that Hatschek 

 has observed during this process the development of the nervous system 

 and the generative organs. The nei'vous system arises as an unpaired 

 thickening of the epiblastic floor of the vestibule, between the mouth and 

 the anus. On becoming constricted off from the epiblast the nerve 

 ganglion contains a central cavity which afterwards vanishes. 



The generative organs originate as a pair of specially large mesoblast 

 cells in the space between the stomach and the floor of the vestibule. 

 These two cells, surrounded by an investment of flattened mesoblast cells, 

 subsequently divide and form two masses. At a still later period each 

 mass divides into an anterior and a posterior part ; the former giving rise 

 to the ovary, the latter to the testis. The similarity of this mode of 

 development of the generative organs to that observed by Butschli in 

 Sagitta, which is described in the sequel, is very striking. 



ECTOPROCTA. 



Although the embryology of the Ectoprocta has been investigated 

 by a very considerable number of the distinguished naturalists of the 

 century, many points connected with it still stand in great need of 

 further elucidation. The original nature of the embryo was rightly 

 interpreted by Grant, Dalyell and other naturalists, but it was not 

 till Huxley demonstrated the presence of both the ovary and testis 

 that the true sexual origin of the embryo in the ovicells became an 

 established fact in science. The recent memoir of Barrois (No. 298), 

 though it contains the record of a vast amount of research, and 



Q 



marks a great advance in our knowledge, still leaves a great number 

 of points, both with reference to the early development and to the 

 larval metamorphosis in a very unsatisfactory condition. 

 Four larval forms can be distinguished, viz. 



(1) A larval form which with slight modifications is common to 

 all the genera of the Chilostomata (except Membranipora and 

 Flustrella) and of the Ctenostomata. 



(2) A bivalved larva of Membranipora known as Cyphonautes, 

 the true nature of which was first recognized by Schneider (No. 322), 

 and the closely allied larva of Flustrella. 



(3) The typical Cyclostomatous larva, for the first full descrip- 

 tion of which we are indebted to Barrois (No. 298). 



(4) The larva of the Gymnolseraata. 



Chilostomata and Ctenostomata. As an example of the first 

 type of larvae, Alcyonidium mytili, one of the Ctenostomata, may be 



1 My view of the metamorphosis which takes place during the fixation of the 

 larva involves the supposition that in Loxosoma, about the attachment of which we 

 know absolutely nothing, two buds are directly formed in accordance with the double 

 nature of the dorsal organ. 



