10. EMBRYOLOGICAL DEVELOPMENT 113 



The internal structure of this embryo oWligobrachia has been insufficiently 

 studied because of the bad state of preservation of the material. Considerable 

 histological differentiation is present in the body wall — there is a thin cuticle, 

 single gland cells may be observed in the epidermis and sparse longitudinal 

 and circular muscle fibres of the dermal muscle sac may be discerned under 

 the epithelium. The transverse diaphragm between the mesosoma and the 

 metasoma is already well pronounced, but its muscle fibres are as yet but 

 feebly developed. 



But as before a large part of the internal space is occupied by the cell mass 

 of the endoderm, not only in the mesosoma and metasoma, but in the proto- 

 soma too, where it begins well forward of the tentacle rudiments. In this 

 archenteric primordium the cells are largely higgledy-piggledy. Only in the 

 protosoma and in the front part of the mesosoma is there some sort of definite 

 arrangement, with large pyramidal cells whose summits meet in the centre 

 (Fig. 8CL4), in such a way that a very narrow space is left between them, 

 perhaps representing the lumen of the enteron. There is no trace of mouth 

 or anus. 



The coelomic sacs look like nothing more than double membranes squeezed 

 between the body wall and the gut primordium. The anterior coelom remains 

 unpaired but takes up an asymmetrical position towards the right side of the 

 body (Fig. 80Л). A coelomic canal runs from it into the rudiment of the first 

 tentacle. 



Blood vessels are already present — at least the dorsal and ventral vessels 

 (Fig. 805). Their walls consist of a thin structureless membrane and their 

 lumina are filled with the small enucleate bodies so characteristic of the 

 blood of these animals. 



Turning now to the bristles of the metasoma, we may observe that each is 

 formed inside a separate multicellular thin-walled sac, so that there are two 

 or three sacs lying in a row in each bundle of bristles. The sacs are situated 

 between the epidermis and the gut rudiment, piercing right through the 

 internal and external walls of the coelom. Each sac consists of three to five 

 cells with round nuclei but with no clear cell walls. The single bristle lies in 

 the middle of this syncytium, resting with its broader end in the epidermis 

 which bulges outwards to accommodate it but is not at first pierced by it. 

 The sac has no connexion with the epidermis. All its constituent cells are 

 identical and bear the same relationship to the bristle (Fig. SI A). It is natural, 

 therefore, to infer that all of them take part in forming the bristle. In any 

 case, we have here no such special formative cell located at the base of the 

 bristle such as we find in the parapodia of Polychaeta. 



