452 COMPARATIVE ANATUMY chap. 



tissue. The blood vascular system of the genital gland may derive its 

 blood in three mutually exclusive ways : (1) by means of a special 

 genital vessel from the iDlood vascular ring, (2) by means of a special 

 genital vessel from the dorsal intestinal vessel, (3) direct from the 

 latter, with which the genital gland is in contact. 



The ventral vessel of the anterior section of the intestine is almost 

 always connected with that of the middle section by means of a 

 usually simple, but sometimes complicated aiiastomosis (Figs. 371, 10, 

 383, 27, p. 477). 



lu the Asjndochiroke principally, but also in many BeuJrocJdrota' 

 and 3Iolpiidiiila', the dorsal vessel becomes detached from the intestine 

 for a considerable distance ; in Hulothuria tuhulosa (Fig. 371) this occurs 

 along part of the anterior,, the whole middle, and part of the posterior 

 section, and it has a free course as the marginal vessel of the rete 

 mirabile through the body cavity. It, however, remains in connec- 

 tion with the lacunar network which is developed in the wall of 

 the intestine by means of a rich plexus of blood lacunse, known as the 

 rete mirabile. This network forms a much perforated membrane, 

 one edge of which is attached to the intestine, while the marginal 

 vessel runs alon? the other. The blood of the intestinal lacunar 

 system, again, may collect in a special longitudinal vessel (collateral 

 vessel, pulmonary vein) before passing over into the rete mirabile 



The rete mirabile is often particularly richly developed in the loop 

 formed by the anterior and middle intestinal sections. In the rest of 

 the small intestine, the vessels which enter the plexus from the marginal 

 vessel first break up into a bundle of very fine lacunae (capillaries) ; 

 these finest lacunse then collect to form vessels which enter the collateral 

 vessel. It may be said that here, between the collateral and the mar- 

 ginal vessels, numerous small retia mirabilia (Fig. 371, 18) of the 

 second order are developed, and that they form webs round the 

 terminal ramifleations of the left water lung. It is doubtful 

 whether they serve for respiration, since they are not developed in 

 the wall of the water lung itself, but merely loosely invest the latter. 



The dorsal vessel (so called because it lies on the anterior intestinal section 

 close to the dorsal mesentery) does not run in this mesentery, but somewhat to the 

 left of it. In the middle section of the intestine it comes to lie on the right, and in 

 the posterior section again on the left side of the mesentery. 



At certain points in the course of the vessel, blood glands may be developed. 

 The spongy, alveolar structure of the vessel then becomes more marked, and many 

 cells are found embedded in the strands, filaments, membranes, etc., of connective 

 tissue which traverse the vessel ; these are the formative cells of the blood corpuscles. 

 At such points the lumina of the vessels (i.e. of the separate lacuiiie, which together 

 form the vessel) are much reduced. 



2. Eehinoidea. — The arrangement of the blood vascular system 

 here agrees to a great extent with that in the Holotlwrioidea. A rich 

 plexus of vacuoles is developed in the connective tissue layer of the 



