Gatty Marine Laboratory, Si. Andreios, 3 



tlie otliers following segmentally, each being joined to its 

 neighbour by fine connectives and giving branches to the 

 muscles and various organs. 



Dr. Thos. Williams'^ (1858) stated that the segmental 

 organs both in Sabellids and Serpulids were absent from the 

 anterior or thoracic region and were present only in the 

 abdominal portion in the form of looped tubes, but he could 

 not distinguish- the part of the tube to which the ova were 

 attached. He thought the ova did not escape into the 

 eoelom, but were confined in a membranous bag. He found 

 a siuiihir structure in the Aaipliictenidae, Spionidse, and 

 other forms. 



In Sjiirofjraphis spallanzani Claii-avhde (1873) describes the 

 giant fibres of tbe nerve-cords as separate in the inter- 

 gauglionic spaces from the trunk, and figures them (his pi. v. 

 fig. 5) surrounded by connective tissue. Internally is a 

 medullary substance. These fil)res run throughout the 

 abdomen without apparent anatomical connection. In the 

 thorax they are repeatedly joined l)y anastomoses. Tlirough- 

 out the rest of the body the uervuus chain is united in each 

 segment by two transverse commissures. On entering the 

 thorax the two tubular fibres divide into two branches, 

 which pass forward reduced in diameter, and ultimately 

 penetrate the cerebral ganglia, where they branch and are 

 lost. Various branches are given off from the tubular fibres 

 along the commissures, but he could not trace them along 

 tlie ventral nerves of the thorax. The branchial nerves are 

 greatly developed in the Sabellids and in Mijxicola. He 

 found in Spirographis that circular muscular fibres penetrate 

 the ventral shields and that the fibres generally show nuclei 

 surrounded by granular matter. Further, that in transverse 

 section of the setigerous processes the bristles are arranged 

 in a spiral, just as Pruvot and liaeovitza showed subse- 

 quently. No dorsal vessel exists anteriorly, only a plexus of 

 anastomosing trunks from which the larjie branchial vessel 

 arises on each side. A periintestinal sinus surrounds the 

 stomach f. There is a well-developed rete in the collar, and 

 the purified blood afterwards enters the ventral trunk. 



He considered that in the Sabellids the connective tissue 

 of the anterior region is of importance and aids in filling up 

 the coelomic cavity, which is almost suppressed, except the 

 spaces for the branchial vessels. The two segmental tubes 



* Philos. Trans. 1858, p. li>3, pi. vii. fig. 13. 



t De Quatrefages first described this plexus arouud the gut (his lacunar 

 system). 



1* 



