1889.] On Phymosoma varians. 123 



structures is the mouth. The space included in the cavity of the 

 tentacular horse-shoe — the representative of the pre-oral lobe — is 

 covered with a peculiar pigmented epithelium, curiously wrinkled. 

 This epithelium is continuous with the brain, and from it two sensory 

 pits descend into that organ. The tentacles are short, the outer 

 surface is grooved and ciliated, the inner surface covered with a 

 pigmented epithelium, similar in character to that covering the pre- 

 oral lobe. 



The introvert immediately behind the head is smooth for about 

 2 mm. At the posterior end of this region a thin but very extensible 

 collar is attached. The anterior end of the collar is free, and in speci- 

 mens with the introvert inverted, completely covers the head. Behind 

 the collar the introvert is surrounded by rings of hooks. Each hook 

 is secreted by a multicellular papilla, and the ring rests upon a 

 cushion formed by circular muscle fibres. Behind each ring of hooks 

 is a parallel ring of sense organs, described below, and underneath 

 each ring is a circular nerve, continuous with the ventral cord. 



The Ectoderm. — The ectoderm covering the lower lip and the out- 

 side of the tentacles is ciliated, that covering the pre-oral lobe and the 

 inside of the tentacle is crowded with brown pigment granules. The 

 ectoderm of the rest of the body is one cell thick. It is curiously 

 vaulted, leaving irregularly scattered spaces between it and the out- 

 side of the circular muscles, in which a nutritive fluid probably 

 circulates. On its outer surface the ectoderm secretes a thick cuticle. 

 From time to time some of the ectodermal cells are modified to form 

 the skin papillae. These are in the form of double cups, the inner lining 

 of which consists of very large cells crowded with yellow spherules. 

 The cup opens to the exterior by a minute pore. These papillae are 

 particularly large and numerous at the extremities of the trunk and 

 on the dorsal surface. Those of the introvert differ from those on the 

 trunk by the possession of a chitinous ring round their orifice. 



General Anatomy. — There is nothing peculiar in the general anatomy 

 of this Grephyrean as seen by the naked eye. Of the four retractors, 

 the two ventral ones are much longer than the two dorsal, and at the 

 base of the former is the generative organ. The intestine is supported 

 by a spindle muscle, as well as by a muscle which passes from the 

 ventral body wall to the anterior end of the visceral loop. There 

 are about twenty-two longitudinal muscles in the middle of the trunk. 

 The body cavity is lined by a flat epithelium, which is never ciliated. 



The Skeletal Tissue. — I have given this name to a peculiar form of 

 tissue which is found in the collar and tentacular crown of Phymosoma. 

 The cells composing this tissue are roundish, with large nuclei. The 

 protoplasm of the cells is traversed by numerous fine lines, and both 

 it and the nuclei readily take up staining fluid. The cells are not so 

 closely packed as to lose their shape. 



