14 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



In Carinella linearis and in Carinomella a very strong band of 

 longitudinal muscles (PI. 8, tigs. 57, 58), continuous with the longi- 

 tudinal muscles of the proboscis sheath more anteriorly, extends 

 beneath the posterior portions of this sheath for some distance into 

 the intestinal region, and eventually fuses with the principal longi- 

 tudinal muscular layer (PI. 9, fig. 59) of the body wall. These 

 muscles are also to be found in otlier species of Carinella and in 

 Carixoma, but in a very rudimentary condition. 



Each muscle iiber consists of a single cell, with an extremely 

 small, slender nucleus, surrounded by an insignificant amount of un- 

 differentiated cytoplasm. This minute cell body lies upon, or imbed- 

 ded in, the fiber of contractile substance. 



Connective 7'issiies and Parenchyma. 



Everywhei'e in the body of the nemertean is a complex network 

 of connective tissue fibers surrounding and supporting the other his- 

 tological elements. In the muscular layers the connective tissue 

 fibers form a considerable proportion of the tissue, and similar fibers 

 make up the great bulk of the cutis. In the cutis of many forms 

 the fibers are imbedded in, and in part replaced by, a closely related 

 gelatinous tissue. 



A somewhat similar tissue fills all the 8i)ace inside the body walls 

 not occupied l)y the viscera, so that a body cavity homologous with 

 that of the annelids is wanting. This gelatinous tissue is termed the 

 body parenchyma, or mesenchyma. 



In some forms there is comparatively little of this parenchyma, 

 the viscera lying closely appressed to the inner l)order of the body 

 walls, l)ut in some other species (as in Amphiporus gelatinosus) the 

 mass of parenchyma in the esophageal region becomes several times 

 as voluminous as the esophagus (PL 19, fig. 120), and continues with 

 but little diminution to the posterior end of the body (PL 20, fig. 

 122). In the head, too, a very thick layer of this tissue separates 

 the brain from the muscular walls of the head, which have but a 

 fraction of the thickness of the parenchyma in the same region (PL 

 19, fig. 119). The great bulk of the substance of the head there- 

 fore consists of parenchyma, and this is l)ut little less true of the 

 other portions of the body. 



As a rule, this tissue is most cons|)icnous in the Iloplonemertea, 



