30 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



sally, laterally, and ventrally, each diverticulum often extending on 

 the ventral side of the intestinal lobe well toward the median line. 

 These diverticula have a special musculature, and although they are 

 commonly narrow and tube-like, can doubtless enlarge so as to con- 

 tain a considerable portion of the fluid of the rhynchocoel. 



The walls of the proboscis sheath are composed of one or more 

 layers of strong muscles (PI. 7, flg. 56) lined internally by a flat- 

 tened epithelium, or more properly endothelium, very much like that 

 of the blood vessels. The endothelium rests on a thick basement 

 layer of homogeneous connective tissue, and in many species of 

 Cerebeatulus, and in some other forms, contains a considerable 

 number of gland cells. These glands are commonly arranged in 

 irregular rows, situated near the rhynchocoel vessels, and pour their 

 secretions into the rhynchocoel. The endothelium is also modified 

 in the vicinity of the dorsal vessel, becoming more columnar and 

 partially specialized into gland cells. 



There are commonly three layers of muscles — outer longitudinal, 

 circular, and inner longitudinal — but in many forms these layers 

 are not very distinct. In Drepanophorus and certain species of 

 Amphiporus there are no distinct layers of muscles in the middle 

 and posterior portions, the musculature consisting of a network of 

 interwoven circular and longitudinal fibers. 



In those Paleonemertea which possess an inner circular muscular 

 layer in the body wall, this layer is sometimes closely bound up with 

 the circular muscles of the proboscis sheath, as described for Cari- 

 NOMA (PI. 13, figs. 81, 82) . 



In many species of Carinella the longitudinal muscles are 

 almost entirely lacking, excej)t ventrally, so that a single layer of 

 circular fibers constitutes almost the entire muscular wall. As a 

 rule the middle portions of the proboscis sheath are thicker than 

 those either in front or behind. 



Rhynchocoel fluid. — The ca"vity of the rhynchocoel is filled with 

 a fluid plasma in which float numerous corpuscles somewhat resem- 

 bling those of the blood, although they are much larger. Each cor- 

 puscle is commonly oval and discoid, although there is mixcli irregu- 

 larity in shape (PL 23, fig. 104). Some are irregularly s])indle-sha])ed 

 and others spheroidal, but all have the power of amoeboid movement 

 and may be seen to form numerous slender pseudopodia from time to 

 time. Several forms mav occur in the same individual. Sometimes 



