40 MORPHOLOGY OP 



The skin of B. Kowalevskii differs in some ways from 

 that of B. minutus, &c., especially that of the trunk, in which 

 the large goblet cells are comparatively rare. In all parts of 

 the skin round, unicellular glands ai'e more or less frequent, but 

 their contents stain more or less deeply with hsematoxylin, &c. 

 These cells often fall out, leaving empty spaces. In the collar 

 of B. Kowalevskii the skin is very thick and is full of very 

 long cells (figs. 80 and 81) containing granular contents, which 

 stain very deeply. 



Fig. 79 shows a section of part of the proboscis skin in 

 which the layer of nerve-fibre is very thick. In the upper 

 part of this kind of skin there is a definite row of long nuclei 

 which with some reagents assume a dice-box shape, probably 

 due to preservation. To what extent these cells reach the 

 whole depth of the skin cannot be affirmed, but many of them 

 can be traced into fibres which run into the layer of nerve- 

 fibres. 



Nervous Concentrations in the Skin. — As has been 

 already mentioned, in all the parts of the skin a greater or 

 less quantity of unstained substance may be found in the base 

 of the skin. The substance contains no nuclei (excepting a 

 few in the nerve-sheath of the base of the proboscis), and may 

 oe seen, especially in fresh osmic acid preparations, to consist 

 of fine fibres. Into it run the tails of ectoderm cells. In the 

 next place fibres may frequently be seen running out of it 

 through the basement membrane, and losing themselves 

 amongst the mesoblastic tissues. The question as to the 

 nature of these fibres is one of great interest. They may 

 either be mesoblastic fibres penetrating into the ectoderm as 

 supporting structures, or they may be epiblastic fibres leaving 

 the skin, in which latter case they are in all probability 

 nervous. 



Somewhat similar fibres have been described by Ludwig in 

 the similar tissues of Asterias, and he is of opinion that they 

 are connective tissue. The possibility, however, that these 

 fibres in Balanoglossus are nervous is supported — firstly, by 

 the fact that they always taper inwards and not out- 



