ON THE DIPLOOHORDA. 289 



drawn the general appearance of the larva intact, and in 

 several positions mainly for the demonstration of several 

 newly described organs. 



PI. 18, fig. 2, gives the appearance of a young Actino- 

 trocha with twenty arms, as seen in side view; the hood is in 

 the position which perhaps may be described as normal. The 

 front parts of a later stage are indicated, as seen from the 

 dorsal surface, in PI. 18, fig. 3; and a side view of the same 

 parts at a yet more advanced stage is seen in PI. 21, fig. 38. 



Lastly, the general shape of the body is shown in PI. 18, 

 fig. 2. This is the side view of a larva with eighteen tentacles, 

 introduced mainly to give an indication of the distribution of 

 the principal nervous tracts. The posture of the pre-oral hood 

 is very characteristic; indeed, most Actinotrochse die in 

 this position, or else with the hood turned still further back. 

 The larva when alive is very active, and on provocation is 

 capable of extraordinary contortions. Figs. 4 and 5 are two 

 outline sketches which indicate two extreme positions assumed 

 by one lively little fellow who lived for some hours in a small 

 glass vessel, the only living specimen I have yet observed. 

 The mobility of the pre-oral hood and its great prominence in 

 the anatomy of the animal are remarkable features of its 

 general anatomy. 



It is not proposed here to follow out the development to 

 any great extent, but there are indications that there is a 

 great increase of size in the later stages, confined mostly to 

 the trunk portion of the body. It is difficult to speak with 

 certainty, but it appears likely that this increase is further 

 followed by a contraction in size, a phenomenon of the same 

 nature as that described for Tornaria by Morgan (17); at 

 least, certain of the larvse are smaller than usual, and in 

 section all their epiblastic cells are closely aggregated, and 

 form a thick epithelial wall, in marked contrast to the greatly 

 attenuated cells which form the outer covering of the trunk in 

 the larger larvae. In these smaller forms the organs are 

 closely packed together, and the coelomic cavities are very 

 reduced. The natural supposition would be that they are the 



