636 THE ANIMAL KINGDOM. 



somewhat flattened or cylindrical in section, and endowed with marvellous 

 contractility. The transparent pelagic Pelagonemertes Rollestoni and 

 the parasitic Malacobdella are short and broad forms. The former has 

 a posterior ventral sucker. 



There is a delicate cuticle, which appears to bear the short numerous 

 cilia ; a cellular ectoderm, in which the cells are of different lengths ; a 

 basement membrane, and a cutis of connective tissue. The basement 

 membrane in Schizonemertea lies between the ectoderm and the cutis, 

 which is largely pervaded by longitudinal muscle fibres ; whereas in the 

 Palaeo- and Hoplonemertea it lies between the cutis and the muscular 

 layers of the body. Many of the ectoderm-cells are glandular, and the 

 gland-cells extend into the cutis. They secrete a plentiful mucus, with 

 which are intermixed minute highly refracting rods. The animals are 

 enabled to creep along the surface of water by means of this mucous 

 exudation, and many, especially those which inhabit mud or sand, protect 

 themselves by a coat or tube of it 1 . The muscular layers of the body 

 are arranged in one of three ways. In the Palaeonemertean genera 

 Carinella and Cephalothrix there is an outer circular, a median longi- 

 tudinal, and an inner circular layer 2 . In the Palaeonemertean genera 

 Polia and Valencinia and the Schizonemertea there is an external longi- 

 tudinal layer of considerable thickness, a median circular and an internal 

 longitudinal layer, whilst the Hoplonemertea possess only an external 

 longitudinal and an internal circular layer. New layers seem to appear 

 in individual specimens of large size, and variations in the mode of 

 preparation cause differences of appearance (Hubrecht). The muscular 

 tissue ofBorlasia is red-coloured, but the source of the colour is not known. 

 There is a plentiful nucleated and partly fibrous connective tissue between 

 the muscle layers and muscle fibres, which appears also to fill the central 

 region of the body, so that there is no coelome 3 . Bands of muscular fibres 

 are disposed dorso-ventrally in the region of the intestine, passing between 

 successive lateral intestinal coeca. Hence the body appears to be divided 

 internally into a series of segments. 



1 The mucus is remarkable for possessing in some instances an acid reaction, in others an 

 alkaline. See Mclntosh, Monograph of British Annelids, pt. i. ; Nemerteans, p. 45. 



2 Pelagonemertes and Monopora are stated to possess an outer circular and an inner longitudinal 

 coat. 



3 The coelome of the larval Lineus obscurus is an archicoele, and is simply a persistent blasto- 

 coele or segmentation cavity. So far as it persists in the adult, it is represented by the cavity of 

 the proboscis sheath and the blood-vascular system (Hubrecht). In the Pilidium, on the contrary, 

 the mesoblast eventually forms two solid masses in the head and two in the trunk ; the former 

 develope some irregular spaces, the future blood lacunae ; the latter splits into a somatic and 

 splanchnic layer. But the cavity between the two is eventually broken up by the growth of cell- 

 processes. In the early stages of Pilidium the mesoblast is chiefly represented by branched cells 

 imbedded in a gelatinous matrix between the epi- and hypo-blast, in which they are freely moveable. 

 These cells appear to give origin eventually to the permanent mesoblast (Salensky). 



