Ou tlie connective tissues and body cavities of tlie Nemertenns. 35 



oii the lateral blood vessels and their coniiiiissures, while the layer 

 around the ventral and lateral surfaces of the proboscis sheath, comes 

 into contact with the dorsal blood vessel, though the latter has not 

 a complete mantle of parenchym cells around it, and is completely 

 devoid of them behind the rhynchocoel; in these species the layer of 

 l)arenchym cells becomes thicker, towards the posterior end of the 

 proboscis sheath, and there forms a sheath around the whole periphery 

 of the latter. The parenchym tissue is apparently absent in the head 

 and oesophageal regions, in all the species ; and also absent around 

 the blood vessels, behind the rhynchocoel, in all except Carinella and 

 Cerehratulus. Of all the genera, this tissue is quantitatively least 

 developed in Stichostemma; and in Tetrasfemma alone do we find the 

 parenchym cells of marked unequal size. Since the parenchym tissue 

 is always present around the dorsal blood vessel (except in Carinella^ 

 where owing to the absence of a dorsal vessel, it surrounds the lateral 

 vessels), and around the proboscis sheath, the parenchym layers of 

 both these organs being in contact, I conclude that the parenchym 

 functionates as a layer for the mutual transfusion of the rhyncho- 

 coelomic with the blood fluid ; and by such a transfusion, the walls 

 of the parenchym cells would probably aid in the osmotic process. 

 And the fluid within the parenchym cells may be, in part at least, 

 fluid of the rhynchocoel or of the blood. 



6) The cells of the intracapsular connective tissue of the nervous 

 systems are in all species, with perhaps the exception of Stichostemma, 

 divisible into two categories: a) those between the outer and inner 

 neurilemma, and b) those around and within the fibrous core, these 

 categories being based on nuclear differences; the group a) may in 

 some species {Cerehratulus lacteus, Amphiporus glutinosus, Tetra- 

 stemma) be divided into two under-groups, in these species a pigment 

 being present in the brain lobes, but not in the lateral nerve chords. 

 The cell fibres of this tissue produce an outer nerve-fibre sheath, the 

 inner sheath, as Bürger has already shown, being formed by the 

 cells of the inner neurilemma. 



7) A noticeable body cavity is undoubtedly present in Carinella 

 and Cerehratulus, and in a more reduced state in Linens gesserensis, 

 Amphiporus and Stichostemma ; so that ray investigations do not agree 

 with Burger's ('95 b), who claims there is no body cavity in the 

 Nemerteans. This body cavity consists of a slit between the intestine 

 and proboscis sheath, and the body wall ; in Cerehratulus it is a very 

 evident cavity, in which both fixed and free mesenchym cells are 



3* 



