THE APODOtrS HOLOTHUKIANS 151 



lacuna? or vessels has any epithelial lining or any other indication of definite 

 walls, save that the intestinal vessels have the superficial appearance of true 

 vessels; the interior, however, is "filled with loose strands and cells of con- 

 nective tissue." The fluid within the system is a colorless plasma with occa- 

 sionally wandering cells, and in the intestinal vessels numerous blood-corpuscles 

 which give the vessels in the living animal a pinkish color. 



Repboducttve system. — The reproductive organs of the MolpadiidiB consist 

 of the same parts which occur in other holothurians — i. e., a genital duct and 

 tufts of genital tubules. The duct opens to the exterior in the mid-dorsal inter- 

 radius just back (1-5 mm.) of the tentacles, and the outlet is often indicated by 

 a more or less prominent genital papilla, of a somewhat conical shape and some- 

 times as much as 2-3 mm. in length. The duct runs downward from the tip 

 of this papilla into the dorsal mesentery, and then backward for some little dis- 

 tance (5-25 mm.) to the point where it divides, and terminates in tufts of geni- 

 tal tubules hanging free in the body-cavity, one on each side of the mesentery 

 (Plate XI, fig. 3). The wall of the duct consists chiefly of connective tissue, 

 lying in which are the longitudinal muscle fibers and wandering cells; outside 

 is the usual flattened peritoneal epithelium, while the interior of the tube is 

 lined with a strongly ciliated, columnar epithelium. In Caiidina arenata (ac- 

 cording to Gerould, '96) this inner epithelium is composed of elongated, 

 spindle-shaped collar-cells, the flagella of which are conspicuously long. At the 

 outer end of the duct is a small sphincter muscle of a few circular fibers, while 

 at the opposite end, where the duct divides, a circular muscle layer outside the 

 connective-tissue layer is present in each branch. The genital duct of the male 

 is usually much smaller near the outer end than posteriorly, where there is 

 sometimes a spindle-shaped enlargement. The genital tubules make up a tuft 

 of 6-8 or more on each side of the mesentery, the number of tubules probably 

 increasing with age. The tuft of the left side is sometimes the larger. The 

 tubules may be very short (5-10 mm.) or long, at times reaching clear to the 

 posterior end of the body-cavity ; they may be simple and undivided or dichoto- 

 mously branched. Their wall is made up of the usual peritoneal epithelium, a 

 thin layer of circular muscle fibers, a layer of connective tissue, in which are 

 conspicuous blood lacuna?, and an inner gei'minal epithelium, which is not uni- 

 form and continuous throughout, but scattered in more or less irregular masses. 

 As the sexes are always separate in the Molpadiids, the product of this epithe- 

 lium will be either eggs or spermatozoa, as the case may be. In Caudina 

 arenata the sexes can be distinguished by the color of the tubules, which, how- 

 ever, is due to their contents and not to any pigment. In the mature male the 

 tubules are light yellow, while in the female they are pale brown. 



