210 DESCRIPTIONS OF PREPARATIONS. 



to the thirteenth somite inclusive, may be seen, on the right side, the cap- 

 sulogenous glands of D'Udekem. These structures appear to be merely 

 enlarged setiparous sacs, and not glands. They have been supposed to 

 secrete the albumen surrounding the ova in the cocoon. The nephridia 

 are left in situ on either side of the nerve-cord in some of the posterior 

 somites. 



The supra-oesophageal or cerebral ganglia belong developmentally to the pro- 

 stomial region of the first somite of the body, but generally shift in the adult 

 backwards even as far in some instances as the 3 rd or 4 th somite. They retain a 

 position in Microchaeta Rappi, &c., in the first (buccal) somite. The oesophageal 

 commissures are composed of one or two fibrous cords. The ventral nerve-cord 

 lies internally to the longitudinal muscular coat, except in the Lumbriculidae, where 

 it lies next to the circular coat. The cerebral ganglia of Aeolosoma are, however, 

 in continuity with the hypodermis, but a ventral cord is absent in this genus. In a 

 few Chaetopoda, in Protodrilus, Polygordius, and Histriodrilus (-=Histriobdella\ the 

 ventral cord as well as the cerebral ganglia are similarly continuous, and the three 

 genera in question have been made into a separate group of Archi-annelidae 

 (Hatschek). Saccocirrus, in which the nervous system is also hypodermic in posi- 

 tion, is in other respects not so archaic a type, and has been classed by Foettinger 

 apart from other Chaetopoda as Archi-chaetopoda. 



The nerve-cord is composed of (i) an external layer of polygonal epithelial 

 cells belonging to the peritoneum; (2) a coat of longitudinal muscle fibres, which 

 does not extend up the commissures to the supra-oesophageal ganglia ; (3) of a 

 neurilemma, formed by the cord itself; and (4) the nervous matter proper, with a 

 neuroglia or supporting connective tissue. The ganglion cells are found on the 

 anterior surface of the supra-oesophageal ganglia, and as a layer on the ventral 

 surface of the ventral cord, but not in the oesophageal commissures. They have 

 no sheaths. In the 'first part of the cord they form a perfectly continuous layer 

 (a fact denied by Vignal), with a right and left and two median aggregations, while 

 in the posterior region these aggregations are separated from one another. This 

 continuous arrangement is found only in Lumbriddae and Lumbriculidae among 

 Oligochaeta. The number of cells does not appear to be much increased in the 

 ganglionic enlargements, which are due chiefly to the greater amount of fibrous 

 matter present at the origin of the -chief nerves. According to Claparede the cord 

 is divided into a right and left half by a median connective tissue septum. 



The nerve-cord has in Lumbriddae three special blood-vessels running longi- 

 tudinally within the muscular sheath, one subneural and two lateral, one on 

 either side. The three are connected by ventral loops just behind each ganglionic 

 enlargement. The lateral vessels give off a branch which accompanies the paired 

 nerves (infra), the median a branch which accompanies each septal nerve (infra), 

 and all three are connected to a capillary plexus ramifying in the substance of the 

 cord and round the ganglion cells. The presence of these vessels was supposed to 

 be a distinctive feature of the Oligochaeta terricola, as opposed to Oligochaeta limi- 

 cola, but the subneural vessel is absent in the terrestrial Megascolex, Perichaeta 

 Houlleti, and Microchaeta Rappi, in the semi-marine Pontodrilus, but present in the 

 aquatic Criodrilus. 



