EARLY DEVELOPMENT OF CRIBRELLA OCULATA. 385 



swimming (cf. Bipinnaria asterigera), and in the latter for fixation, but there is little 

 to choose in the matter of ' directness ' of development between the two types. The 

 term ' larva' has been applied freely to these free stages, but it may be noted that its 

 use will to many appear to be unjustifiable. If the difference between an 'embryo' 

 and a ' larva ' is to be defined in terms of alimentation, and a larva be defined as a young 

 organism which obtains its own nutriment, then Cribrella has no larva at all, and 

 its development is purely embryonic. A similar remark applies to the early tadpole 

 stages of the frog and to the young Amphioxus. If, on the other hand, we define the 

 larva by the criterion of locomotion, a larva being a young animal capable of indepen- 

 dent and free movement, then the young stages of Cribrella here described are true larvae. 

 Although the former definition has been customary, I think that for many reasons the 

 latter recommends itself as being more easily defined and a more natural distinction. 



Internal Structure. (Stages A-D.) 



Stage A. — A nearly median horizontal section (fig. 29) through stage A reveals a 

 structure closely similar to that of the last embryonic stage, but the hypoblastic sac 

 has become isolated and is assuming definite shape. A constriction towards the centre 

 gives it a dumbbell shape. The anterior portion thus defined may be termed the 

 anterior ccelom ; its walls are already perceptibly thinner than those of the rest of 

 the hypoblastic sac. Dorsally it grows backwards slightly over the constriction, and 

 on either side it is produced backwards into processes which are precisely similar. 

 The left of these horns will produce the lining epithelium of the water-vascular system, 

 and from its position we may recognise it as the left lateral coelom. Its fellow is the right 

 lateral coelom. The middle constriction becomes in later stages the true mesenteron, and 

 the posterior portion we may term the posterior coelom : it also shows slight indications 

 of right and left horns, which later give rise to the right and left posterior cceloms. A 

 more ventral section shows the epiblastic pit, the last trace of the blastopore, and it also 

 cuts the anterior and posterior cceloms separately. Histologically there is little change 

 from the last embryonic stage. The hypoblast cells are more numerous and more 

 clearly defined, and their arrangement more definite. As in the embryo, the archenteric 

 cavity is completely filled with hypenchyme of the same reticulate nature. 



Stage B — (figs. 30-35) — shows several important advances upon stage A. As is to 

 be expected, the anterior ccelom is now large and expanded anteriorly, the two lateral 

 cceloms (figs. L. 2, R. 2, 30, 31) are now sharply defined, and dorsally their cavities 

 communicate only by a narrow canal with that of the anterior ccelom. In the median 

 dorsal line the posterior wall of the anterior ccelom commences to push backwards as 

 a small vesicle, not unlike the hydrocoele, but smaller (fig. 31, c) : it is here called the 

 central coelom. From this small pouch (which becomes the cardiac vesicle (' herzblase,' 

 pericardium) of the adult), there passes a groove downwards (fig. 32) to the mesenteron, 

 which is still merely a canal connecting the two cceloms. At the level of the mesenteron, 

 the lateral cceloms are still widely open to the anterior ccelom. In fig. 34 is still seen 



