248 COMPARATIVE ANATOMY CHAP. 



XL Blood-Vascular System. 



There is no organic system in the worms which is so variable as 

 the blood-vascular system. It is sometimes wanting, sometimes highly- 

 developed. We even find that it may be wanting in certain groups 

 whose nearest relations possess it. Thus, as we find a blood-vascular 

 system sometimes present, sometimes absent, in the most different 

 orders of the most different classes, its small morphological worth is 

 evident. Even where worms are supplied with a circulatory apparatus 

 it varies so much in structure that no morphological comparison is 

 possible, at any rate at present. 



Nemertina (Fig. 157, p. 235). In this class of the Fermes 

 for the first time in the animal kingdom we meet with a blood- 

 vascular system. In the Paleonemertina (excepting Falendniidce and 

 Polliidce) it consists of 2 lateral vascular trunks, uniting posteriorly 

 above the intestine, and anteriorly entering a lacunar system, 

 which likewise establishes communication between them. In the 

 Schizwiemertina (with the Polliidce and Valendniidce) there are 3 longi- 

 tudinal vessels, 2 lateral, and 1 medio- dorsal which lies above the 

 intestine, in the proboscidal region between the proboscis and the 

 intestine. The 3 vessels become lacunar anteriorly, and communicate 

 above and below the proboscis sheath. In the rest of the body they 

 are connected together by transverse vessels. The same is the case in 

 the Hoplonemertina, only a lacunar system is here wanting, and the 

 vascular system is completely closed. The blood is colourless or 

 contains red blood corpuscles. The vessels are lined with endothelium 

 and occasionally have muscular walls. 



Nemathelminthes. The Nematoda have no vessels. In the 

 Acanthocephala, throughout the whole subcuticle, a peculiar network 

 of canals extends, whose morphological and physiological significance is 

 still little understood. The system of canals, which are hollow spaces 

 without walls of their own, running in the very much thickened 

 subcuticle, consists of 2 completely separate parts the canal system 

 of the trunk and the canal system of the neck, the proboscis, and 

 the lemnisci. In the trunk canal system we find 2 specially distinct 

 longitudinal trunks which run either laterally, or dorsally and ven- 

 trally. The neck, head, and lemniscal canal system enters a circular 

 canal situated at the base of the neck. The lemnisci (Fig. 172, /, p. 

 258 ) are two pouches, generally of a brown colour, which hang from 

 the base of the neck into the body cavity and are processes or 

 appendages of the subcuticle of the neck. A canal enters each 

 lemniscus from the circular canal, and divides into two branches 

 directly after entering ; these branches run longitudinally through the 

 lemniscus. Besides these, narrower canals also occur in the lemnisci. 



Annulata. The Hirudinea and the Chcetopoda are separated by 

 sharp and radical differences in the blood-vascular system. In the 

 Myzostomidce blood-vessels are altogether wanting. 



