74 ARTHUR E. SHIPLEY. 



of rock which were completely covered at low water contained 

 many more specimens than those which were left dry by the 

 tide. 



The species seems to be capable of much variation ; and the 

 descriptions hitherto published are incomplete in one or two 

 important points. A detailed account of the external cha- 

 racters may therefore be not altogether useless. 



External Characters and Ectoderm. 



The length of fully extended specimens averages 50 mm., 



varying, however, from about 40 mm. to 55 mm. The greatest 



diameter of the trunk is from 4 mm. to 5 mm. ; that of the 



introvert about 2 mm. The introvert is at leastr equal in length 



.to the rest of the body. 



The head (figs. 1 and 5) bears a crown of about eighteen 

 tentacles, arranged in the form of a horseshoe, with the open 

 ends directed backwards ; the whole structure lying far back 

 on the dorsal region of the head (fig. 1). The ends of the 

 tentacular horseshoe are connected with the lower lip; which 

 is a thick vascular crescent enclosing considerably more than 

 three-fourths of the circumference of the head (figs. 2 and 6). 

 The mouth is a narrow crescentiform slit, extending between 

 the dorsal margin of the lower lip and the convex surface of 

 the crown of tentacles. These relations of tentacular crown, 

 mouth, and lower lip are shown in the diagram (figs. 1 and 32). 

 It will be seen that in this species the condition of the head 

 presents a marked resemblance to that which obtains in 

 Phoronis. 



The tentacles themselves are short and simple, the surface 

 directed towards the outer (convex) side of the lophophor 

 being grooved, and the groove is ciliated ; the opposite surface 

 is covered with a deep brown pigmented epithelium (fig. 5). 



The space included within the concavity of the lophophor 

 (the representative of the pr£eoral lobe) is covered with a 

 wrinkled, pigmented skin. In its centre lies a deep depression, 

 similar to that of Sipunculus, at the base of which lies the 



