254 COMPARATIVE ANATOMY chap. 



vascular system with red blood corpuscles. A dorsal blood-vessel 

 conducts the blood into a vessel which runs along the base of the 

 tentacle crown and gives off branches to the tentacles. Other 

 branches conduct the blood back into a second vascular ring which runs 

 on the outer side of the former. A vessel rises from each side of the 

 external vascular ring, which unites with that from the other side 

 under the oesophagus, and runs back as ventral vessel asymmetrically 

 in the left chamber of the body cavity; this vessel has numerous 

 lateral cseca. Besides these vessels there is a blood sinus around the 

 stomach intestine. All the vessels are contractile. 



The opinions of the most recent investigators as to the blood- vascular 

 system of the Brachiopoda vary greatly. Some deny the existence of 

 any circulatory apparatus. There is only, they say, a system of sinuses 

 belonging to the body cavity. Other investigations confirm the old view 

 according to which, in some Brachiopoda a contractile tubular heart lies 

 above the stomach, and a vein arising from the heart over the fore-gut. 

 There are also said to be vessels in the arms, and also vessels called 

 genital arteries. 



A blood-vascular system is wanting in the Rotatoria, Dinophilus, 

 and the Cluetognatha. 



XII. Genital Organs. 



Division of the sexes generally prevails among the worms. The 

 exceptions to this rule, apart from single cases, are the Hirudinea, 

 Oliffoclueta, Myzostomidce, Chcetognaiha, Phoronis, and many Bryozoa. 



Nemertina. — The genital apparatus (Fig. 135, p. 205) is here very 

 simple. The ovaries in the female and the testes in the male are 

 present in large numbers, and are found in the shape of small sacs in 

 adult animals throughout that region of the body through which the 

 mid-gut runs. They lie in the parenchyma (jelly) under the mus- 

 culature. Each ovary and each testis, at the time of sexual maturity, 

 becomes directly connected with the exterior by means of a special 

 duct. The genital glands on each side generally lie in a longitudinal 

 row in such a way that between two consecutive diverticula of the 

 intestine there always lies an ovary or a testicle. They are therefore 

 more or less regularly metameric in their arrangement, in cor- 

 respondence with the more or less regular metameric arrangement 

 of the enteric diverticula themselves. In some Nemertina there are 

 also genital glands scattered about the parenchyma, each gland, however, 

 has an independent external aperture. In this arrangement we find 

 great agreement with the Turbellaria (especially the Polydada and 

 Tridada). In the latter, however, the oviducts and sperm ducts 

 arising from the genital glands unite to form common channels of exit. 



Nemathelminths. — 1. Nematoda. — The male genital apparatus is 

 unpaired, and emerges at the posterior end of the body in the cloaca ; 

 the female apparatus is paired, and emerges externally on the ventral 



