ii6 



ZOOLOGY 



powers of contractility, but are very easily broken into frag- 

 ments. 



Tetrastemma flavida, which may serve as a type of the 

 class, is a pinkish Nemertine an inch or more in length, which 

 is found under stones or in fissures of the rock when the tide 

 is down. It occurs in all European waters from Scotland to 

 the Eed Sea ; it is extremely delicate, and the slightest touch 

 may serve to break it. 



The ectoderm of this animal is columnar and ciliated. 

 Beneath the ectoderm, and separated from it by a basement 

 membrane, is the thin layer of circular muscles, and this layer 

 in its turn surrounds a thickish layer of longitudinal muscles 



FiQ. 76. — ^Diagrammatic section to 

 show disposition of parts in a 

 Hoplonemertean (such as Tetra- 

 stemma). After Hubrecht. 



1. Cellular portion of integument. 



2. Basement membrane. 



3. Circular muscular layer. 



4. Longitudinal muscular layer. 



5. Ijateral nerve. 



6. Cavity of proboscis sheath. 



7. Proboscis. 



8. Intestine. 



9. Lateral blood-vessels. 



10. Dorsal blood-vessel. 



11. Connective tissue. 



(rig. 76). From the latter, fibres arise and pass in various 

 directions through the body, certain of these are especially 

 conspicuous, running in a dorso-ventral direction between the 

 diverticula of the alimentary canal, and these give the body 

 the appearance of being segmented, but the bundles of muscles 

 are not very regular, nor are they always opposite each other. 

 A layer of diagonal muscle-fibres usually occurs within the 

 longitudinal. 



The integument of Nemertines encloses a more or less 

 solid mass of parenchyma, in which the various organs of the 

 body are embedded. In this there are two kinds of spaces. 

 Firstly, there is the space into which the proboscis is with- 

 drawn, and the blood-vessels. These contain a fluid in which 

 corpuscles float, and they represent that kind of coelom known 

 as an archicoel. And then there are the spaces in which the 



