GEPHYREA i6i 



Phymosoma varians is a West Indian species found em- 

 bedded in the soft coral rock, in which it bores tubular 

 passages, probably dissolving the soft rock by some chemical 

 excretion. Its colour is brownish-yellow. In its extended 

 condition it is about 5 cm. long and about ^ cm. broad, 

 tapering at each end. The anterior half of the body, the 

 introvert, can be withdrawn into the posterior half, just as 

 the finger of a glove can be invaginated into the hand. 



The mouth is terminal, and is at the end of the introvert. 

 Dorsal to the mouth is a crown of eighteen or twenty short 

 tentacles arranged in a horse-shoe, the lophophore (Figs. 103 and 

 1 5). The dorsal ends of this horse-shoe are continuous with 

 the dorsal ends of a thickened lower lip, between which and 

 the crown of tentacles or lophophore the mouth opens. The 

 mouth has therefore the form of a crescentiform slit. In 

 the hollow of the horseshoe-shaped lophophore the skin is 

 wrinkled and pigmented ; close beneath it, and in direct con- 

 tinuity with it, lies the bilobed supra-oesophageal ganglion. 

 About 2 mm. behiud the mouth, a very extensile fold of tissue 

 forms a riag-hke collar round the base of the head. This 

 collar can be produced so as to cover in the whole head. 



The introvert is distinguished from the rest of the body 

 by the presence of numerous rows of minute chitinoid hooks 

 (Fig. 103), which alternate irregularly with certain papillae 

 to be described below. 



The integument consists of the following layers : (i.) the 

 ectoderm, (ii.) circular muscles, (iii.) longitudinal muscles, and. 

 (iv.) peritoneal epithelium. The ectoderm is a single layer of 

 cubical cells. Those covering the lower lip, and that side of 

 the tentacles turned towards ,.^e mouth, bear cUia. The 

 ectoderm of the concave side of the lophophore and its hollow 

 is crowded with black pigment, and at two places it is con- 

 tinuous with the substance of the brain. Over the rest of the 

 body the ectoderm secretes a thick cuticle, which is only 

 broken by the presence of the skin papiUae. 



These papillae are very characteristic of the Gephyrea ; 

 they are formed by the ectoderm becoming folded into the 

 shape of a double narrow-mouthed conical cup. The outer 

 layer of cells resembles the ordinary ectoderm ; the inner, how- 



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