218 MORPHOLOGY OF INVERTEBRATE TYPES 



9. Hold the specimen, in water, on edge, ventral side up, and 

 press down the outer left gill. Notice between the outer and 

 inner gill a white line. This is the attachment line of the gills 

 to each other and contains the branchial artery. Make a drawing 

 showing the two gills. Label also the gill-filaments and ridges 

 or folia. 



10. Remove both gills. Place a piece of a gill on a slide and 

 cut it with a razor at right angles to the filaments by pressing 

 the razor against it (not by drawing it). Make now in the same 

 manner an incision parallel to and close to the first, thus ob- 

 taining a cross-section of the gill-filaments. Examine under 

 microscope (50 diameters) in a drop of water without cover glass. 

 Make a drawing showing three ridges or folia of the two lamellae 

 with their filaments, interfoliary ridges, water tubes and blood- 

 vessels. 



11. Holding the animal on edge, ventral side up, find the 

 yellow visceral ganglion under the posterior retractor of the foot 

 with nerves forming a letter X. The two nerves running back- 

 ward are the pallial nerves of the siphon. The two anterior 

 arms of the X are formed by two pairs of nerves. Of these the 

 outer pair soon bends backward and enters the gills. This pair 

 of nerves is called branchial gill nerves. The two inner anterior 

 nerves are the cerebrovisceral connectives. 



12. Remove both labial palps. Split the foot with a razor 

 longitudinally into two symmetrical parts. Cut off the left half 

 by means of an incision parallel to the attachment line of the 

 gill and running backward from the middle of the anterior 

 adductor. Remove the left half of the foot and find on the cut 

 surface of the right half the pedal ganglion a little yellow or 

 pink body at the base of the foot. Remove the left wall of the 

 pericard and of the urinary bladder. Make a full size drawing 

 showing outline of shell; edge of mantle; both adductors of the 

 valves; both retractors of the foot; left retractor of siphon; left 

 cerebropleural ganglion, which has the appearance of a little 

 yellow or pink body under the anterior retractor; pedal ganglion; 



