466 J. F. ILLINGWORTH 



The gills are symmetrical, and lie on either side of the mantle 

 cavity. If we make a median, longitudinal section through the 

 ctenidium we will see that the laminae are placed on each side of a 

 central chamber in a pinnate arrangement, A cross-section cuts parallel 

 to the laminae, and shows the afierent and efferent branchial sinuses. 



The blood, which has been distributed by the arterial circulation 

 is collected as follows : First, that of the foot is collected by a mesh- 

 work of veins lying on the floor of the visceral cavity (Fig. 3). The 

 finer collectors are united into larger trunks of which there are four, 

 two lateral {Lat), and two larger median (Med). The last are some- 

 times almost fused together. All these collectors carry the blood 

 toward the anterior end of the visceral cavity. The lateral ones run 

 up along the wall and curve back toward the bases of the ctenidia. 

 Here they enter the large basi-branchial sinus which (Fig, M Bas) 

 acts as a reservoir. It is just in front of the pericardial cavity, and 

 below the rectum, the left kidney being in its posterior wall. The 

 median collectors from the foot have numerous branches running over 

 the walls of the oesophagus. All of these are united into a single 

 vein, which passes up around the right side of the oesophagus and 

 opens into the basi-branchial sinus. The blood from the viscera is 

 collected by numerous small vessels and brought to the right ne- 

 phridium, through which it passes before entering the basi-branchial 

 sinus. 



The circulation of Lucapina will be made clearer by the following 

 diagram (see page 467). 



The Nervous System. The large size oi Lucapina crenulata 

 makes it possible to dissect out many nerves that have been seen 

 only in sections of other species of Fissur ellidae. This has led to the 

 discovery of some interesting relations which will be described in the 

 following paragraphs. 



Each cerebral ganglion is situated just within the base of the 

 tentacle. By careful examination after clearing in oil, it is found that 

 they are of smaller size than might at first appear for the labial 

 ganglion which is attached to the inner side of each is well developed. 

 The relation of the two is partially brought out in PI. 33, Numerous 

 nerves are given off from both cerebral ganglia, the first, the cerebral 

 commissure (Cer.c) connecting two ganglia from their anterior ends. 

 On the outside of the ganglion near the front end, there are two 

 nerves (H.w) going to the head walls. Back of these on the same 



