58 THE APODOUS HOLOTHTJKIANS 



BLOOD SYSTEM. In connection with the digestive canal, there are present 

 in the Synaptidae two long longitudinal vessels, which form the principal por- 

 tion of the blood system (Plate VI, fig. 6). One of these lies just at the line of 

 union of the mesentery and alimentary canal, and is called the dorsal vessel, 

 while the other is on the opposite side, and is called the ventral vessel. These 

 vessels may be closely appressed to the stomach and intestine or they may be 

 completely separated from them by a greater or less distance. In the former 

 case they are connected with each other by lacunar spaces in the wall of the 

 alimentary canal, but when separated from the latter they are connected with 

 its wall by short cross-vessels which open into the lacunar spaces or into a 

 secondary longitudinal vessel from which lacunae arise. The ventral blood- 

 vessel runs from the lower end of the oesophagus back as far as the middle of 

 the third section of the alimentary canal, both ends fading out into lacunar 

 spaces. Between the ventral blood-vessel of the first and second sections of the 

 digestive tube (sometimes between second and third, sometimes even between 

 first and third) there are usually present one or more connecting vessels of 

 considerable prominence. The dorsal vessel runs backward about as far as the 

 ventral vessel, but anteriorly it extends to the water-vascular ring, upon the 

 inner surface of which there lies the circumoesophageal ring of the blood-sys- 

 tem, in which it terminates. Direct connection between the ventral vessel and 

 this ring has yet to be demonstrated in the Synaptidae. From the blood-ring 

 arise vessels running into each tentacle along the inner surface, close to the 

 tentacle canals. The genital gland receives its blood-supply directly from the 

 dorsal vessel. Eadial blood-vessels are wanting in the Synaptids. The blood 

 is usually colorless, but in some of the larger species it is yellowish. It con- 

 tains much more albuminous material than the fluid of the water-vascular sys- 

 tem; it also contains corpuscles, amoeboid cells, and minute granules. 



REPRODUCTIVE SYSTEM. The reproductive organs of the Synaptidae consist 

 of a more or less branched pair of tube-like projections, one on each side of the 

 dorsal mesentery, in which lies the common genital duct, into which they open. 

 The duct is short and opens to the outside by a single pore, in the mid-dorsal 

 interradius, just back of the circle of tentacles. In Synaptula hydriformis, 

 however, the duct is not united with the outer epithelium of the body-wall and 

 probably communicates with the exterior by several minute pores. The genital 

 tubes themselves lie in the anterior part of the body-cavity ; there is rarely only 

 a single branch on each side, generally several or many branches. Usually the 

 branches or tufts of branches of the two sides are of equal size in adult 

 Synaptids, but in the young, so far as is known, that of the left side is the 

 smaller, and this asymmetrical condition may persist in some adults. The tubes 

 are often somewhat colored, usually yellowish, sometimes green, but more com- 

 monly they are white, although the shade varies with the condition of the con- 



