No. 471] ANATOMY OF ACMJEA TESTUDINALIS 



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be seen small opaque patches, the expression of interspaces be- 

 tween vessels that open directly into the auricle or the gill vessels. 

 Those which open into the afferent vessel come from the anterior 

 part of the suprarenal plexus; those which open into the auricle 

 and the efferent vessel come from the nuchal plexus. 



The nuchal cavity (Fig. 3), as has been said, lies in front of the 

 visceral sac and above the head and neck; it is somewhat triangu- 

 lar in longisection and its posterior wall curves from side to side so 

 that the cavity is much deeper from front to back in the median 

 line than laterally. It contains the following structures, which 

 with the exception of gill and pericardium are borne entirely on 

 the posterior wall : pericardium with the enclosed heart, ctenidium, 

 papillae of small left and of large right nephridia, anal papilla. 

 Separate generative openings are absent, as is also a hypobran- 

 chial gland. 



On looking into the cavity from the front, one notices first the 

 large ctenidium whose line of attachment runs along the mantle 

 from the left tip of the columellar muscle obliquely back to the 

 hinder wall of the cavity, where it ends a little on the right of the 

 median plane. Through the thin posterior wall of the chamber 

 can be seen the rectum lying near its dorsal edge and extending 

 from the ctenidium almost to the right tip of the columellar muscle, 

 where it ends upon a prominent anal papilla. Below the rectum 

 appears ^ portion of the large right nephridium. It opens by a 

 sizable papilla (infra-anal papilla of authors) located at the right 

 of the anus. Above the rectum, in the triangle included between 

 it, the gill, and the dorsal edge of the mantle cavity, lies the small 

 left nephridium; it opens by an inconspicuous papilla (supra-anal 

 papilla of authors) above and to the left of the anus. Behind and 

 on the left of the ctenidium is a large triangular space enclosed 

 between it and the columellar muscle and lying partly in the poste- 

 rior and partly in the dorsal wall; this is the pericanliiun. Tlie 

 osphradia are so inconspicuous a,s to be readily ovnlookrd ; tlu'v 

 are a pair of narrow transverse epithelial ridges which on the 

 neck a little behind the anterior end of the columellar muscle. In 

 a specimen whose shell was 35 mm. in length the left osphradium 

 was 2 mm. long and the right 1.5 mm. According to Dall these 

 structures are sometimes rendered conspicuous by an orange pig- 

 ment; I have never seen such specimens. 



