112 



PART I. GENERAL ACCOUNT 



Later stages of development 



The next known stage of development has been recorded from Oligo- 

 brachia dogieli. Segmentation of the body has already proceeded to comple- 

 tion and tentacle rudiments are present, so that it is at last possible to 

 distinguish dorsal and ventral sides. The body of the embryo is already much 

 elongated and almost cylindrical, and plainly divided into three segments 

 (Fig. 79). The protosoma is by no means small; its front end is conical and 

 drawn out to form the cephalic lobe. It is divided from the mesosoma behind 

 by an annular groove. The much thickened epidermal epithelium on the tip 

 of, the cephalic lobe consists of tall cells, but elsewhere on the protosoma it is 

 more or less thin. A considerable part of it is densely covered with the short 

 cilia of the anterior ciliated band. 



The tentacles are found on the ventral side of the protosoma, just in front 

 of its posterior edge. A small tongue-like outgrowth situated a little to the 

 right of the mid-ventral line is the rudiment of the first tentacle (Fig. 79.) 

 Quite soon a small wart-like thickening of the ectoderm may be observed to 

 its left. This represents the rudiment of the next tentacle to be formed — the 

 first left tentacle — thus completing the first pair. 



The mesosoma has already undergone considerable growth in length and 

 now makes up a large part of the larva. There is no sign yet of the bridle. 

 Almost the whole dorsal surface of the front half of the mesosoma is covered 

 with short cilia. 



The metasoma is very small and more or less egg-shaped. It is only about 

 \\ times as long as it is broad. It is divided from the mesosoma by a sharply 

 nipped-in waist. A narrow ciliated band is de- 

 veloped on the surface of the metasoma with a few 

 bristles lying in front of it. The latter form a bundle 

 on each side of the body, consisting of two or 

 three rather thick short bristles which stick out. 

 The ends of the bristles are slightly swollen and 

 rounded, and they point sideways and somewhat 

 forwards (Fig. 79). They consist of a clear trans- 

 parent substance and reach a length of 48/x. 



— ms 



Fig. 79. A segmented embryo of Oligobrachia dogieli, at the stage 

 of the first appearance of tentacles, seen from the ventral side. 



ms - mesosoma; mts - metasoma; ps - protosoma; s 1 - groove 

 between protosoma and mesosoma ; s 2 - groove between the meso- 

 soma and the metasoma ; td - first tentacle ; ts - bud of second 

 tentacle. (After Ivanov, 1957b.) 



