40 Proceedings of the Royal Fliysical Society. 



the gastric tube may be cut twice or even thrice, instead of 

 once. Such a condition is seen in Fig. 4 on PL III., where 

 the wall of the gastric tube appears three times in one part 

 of the section. 



In a section lower down, below the gastric tube, the inner 

 ends of the mesenteries are free, but thickened and con- 

 voluted, as will be described further on. 



B. — Histology. 



The histology of the different parts of the animal must now 

 be considered in detail. The ectoderm is covered in the non- 

 retractile part of the body wall by a delicate structureless 

 cuticle. Gosse ^ inferred that the integument was of a 

 chitinous nature, from the way in which the dead polype 

 retained its form. The cuticle is clearly visible in most of 

 my sections through the non-retractile portion of the body 

 wall ; and I have succeeded in some places in tearing it off, 

 so as to have it in the isolated condition. At first I thought 

 it was confined to this non-retractile i)art of the body wall, 

 but as I afterwards found a delicate layer visible covering 

 the ectoderm lining the upper part of the invaginated tube, 

 it is probable that it is secreted over the whole surface of the 

 ectoderm, though possibly it is much thicker on the non- 

 retractile body wall than elsewhere. I have not been able to 

 find any trace of it on the tentacles or in the gastric tube. 

 The cuticle appears perfectly structureless, and is of a pale 

 brown colour, staining slightly yellow with picro-carmine. 



The ectoderm below the cuticle is formed of a single layer 

 of cells, which in section (see PL II., Fig. 8, ec.) are elongated 

 and spindle-shaped, with distinct nuclei in the wider central 

 portion. In surface view they are large polygonal squames, 

 with granular contents and distinct centrally-placed nuclei 

 (PL II., Fig. 7, ec). They stain light pink with picro-carmine. 



The mesoderm of the body wall is a thick layer of tissue, 

 which seems intermediate between a gelatinous and a cartila- 

 ginous condition. It consists of an apparently structureless 

 matrix, containing scattered through it small rounded and 



1 Loc. cit., p. 278. 



