330 ME. A, E. SHIPLEY ON THE GEIJTJS SIPUNCULirS. [Apr. 18, 



Siimncidus nudus they are finger-formed ; in Sipunculus tessellatus 

 they are fused into a membrane at their base and at their free 

 ends are somewhat branched. In the last-named species they 

 bear certain pigmented spots, which are shown in Plate XXV. 

 fig. 3. They project into the body-cavity between the dorsal 

 retractor muscles on the dorsal aspect of the brain. 



Sections throw but little light on the nature of the structures. 

 They are covered by a layer of cells continuous with those covering 

 the brain, and they are not ciliated. They are solid and consist 

 of a number of connective-tissue cells, and they are rather richly 

 supplied with nerve-fibres. Beyond the fact that they receive a 

 somewhat more abundant nervous supply than other organs there 

 is nothing in their structure to suggest tliat tliey are sense-organs, 

 and 1 am quite unable to surmise what their function may be. 



Plate XXVII. fig. 13, which was drawn with the view of showing 

 the position of these structures, is a section near the dorsal middle 

 line of the fringe-like lophophore of S, nudus. It shows the 

 relation of the brain to the dorsal blood-vessel, the ventral half of 

 the central nervous system being bathed by the blood in this tube. 

 The dorsal blood-vessel is continued on into the lophophore and 

 breaks up into numerous lacunae, which when charged with blood 

 serve to extend these parts. The same figure shows the attach- 

 ment of the retractor muscle to the base of the brain, the giant 

 ceUs in the latter, and the area whei'e the brain is continuous with 

 the epiderjnis, and the ciliated pit leading to this. This latter has 

 been very accurately and fully described by H. B. "Ward luider the 

 name of the cerebral organ. 



TJie Rectal Diverticula. 



There are in most members of the genus Sipunculus two 

 feathery structures attached to the rectum in the immediate 

 neighbourhood of the anus. These, like the papdlse on the brain, 

 project into the coelom and are bathed on all sides by the ccelomic 

 fluid. If a small piece of these bodies be examined under the 

 microscope it is seen to have a very branching, frayed appearance, 

 somewhat resembling a minute sea-weed (Plate XXVII. fig. 12). 

 The terminal branches are somewhat swollen, and show no trace 

 of any pore or opening, such as are found in the terminal branches 

 of the anal cseca of Bonellia and some other armed Gephyrea. 



The following description applies to these organs in the species 

 Sipunculus nudus and tessellatus. In stained specimens the nuclei 

 are very prominent ; they are apt to stand out from the surface of 

 the structui'es, and in some cases seem to have absolutely separated 

 off from the body of their cells. Whether this is really the case 

 or not I cannot say, as I have not been able to examine fresh 

 material, and that which I had at my disposal was not well enough 

 preserved to permit the determination of this point. 



The branches gradually collect together and end in one main 

 trunk, and this opens, not, as I expected to find, into the alimentary 



