ANATOMY AND DEVELOPMENT OF THE ECHINODERMS III 



the more or less radiate adult Echinoderms are developed by a 

 process which is a sort of internal gemmation. 



Now the result of this process is twofold ; either the new structure 

 ultimately throws off more or less of the larva in which it was 

 developed, or it unites with the larva to form the adult animal, no part 

 being thrown off 



The former is the case in the Ophiuridae, Echinidae and Asterida;, 

 for the most part — the latter in the Holothuriadae. 



The latter process, as the simpler, shall be described first. 



A portion of the dorsal integument of the larva becomes as it were 

 thrust inwards (fig. lo) towards one or other side of the stomach, as a 

 tube terminated by an enlarged globular extremity, whose cavity 

 communicates with the exterior and is ciliated internally. 



The vesicle which terminates this " internal bud " now sends forth 

 processes so as to form a sort of " rosette," which lies close to and 

 above the stomach. 



The " rosette " becomes a circular canal (the circular canal of the 

 water-vascular .system), from which ca;ca are given off anteriorly to 

 form the tentacles, posteriorly to the parietes, in which they become 

 the water-canals. 



The former mouth of the larva is obliterated, and a new one is 

 formed in the centre of the circular canal and its tentacular appen- 

 dages. This is the permanent mouth of the Holothuria, which is 

 therefore a new structure forined upon the dorsum of the larva. 



In the meanwhile, vesicles, the Vesiculae PoliauK, are developed 

 from the circular canal, and a deposit of calcareous matter takes place 

 round a portion of the tubular canal, from ^\'hose spherical extremity 

 the water-vascular system has been formed. That portion of the 

 tubular canal which lies between the dorsal parietes and the calcareous 

 deposition dies away, and the remainder hangs freely from the 

 circular canal of the water-vascular system as the " sand-canal." 



The process in the Echinidae, Asteridae, and Ophiuridae is essen- 

 tially the same ; only, as in these the old body is to be more or less 

 completely discarded, the development of the water-vascular system is 

 attended, pari passu, by that of a mass of cells from which the new 

 body is to be formed. 



We cannot do better than adduce in illustration Prof Mtiller's de- 

 scription of the development of the Echinoderm in the Asterid-larva 

 Bipinnaria (Fortsetzung der Untersuchungen iiber die Metamorphose 

 d. Echinodermen, Miill. Archiv, 1850). 



In larvae which are not 0'I5 of a line in length, the dorsal pore and 

 the tube which proceeds from it are perceptible. It passes into a 



