THE INVERTEBRATE COURSE. 



129 



3. The heart 1 and its vessels are more or less exposed when the 

 carapace is removed. Carefully free the pericardial area from 

 gonad, etc., preserving if possible the frontal artery, the aortic 

 arches, and the lateral arteries. In some cases the collateral 

 arteries and their fusion to form the superior abdominal artery 

 can be seen. Some of the branchio-cardia canals, which return 

 blood from the gills to the pericardium, can also be identified. 

 Note also the median cardiac nerve and make out as much as pos- 

 sible of the heart itself and its eight pairs of ostia. Then have 

 your dissection demonstrated by an instructor. (The vascular 

 system is not well shown in such fresh material, but the main 

 features can usually be indicated.) 



4. Remove the heart exposing the digestive system. Clear up 

 the neighborhood in the same way as for the vascular system. 

 Take care to preserve the large ducts by which the liver com- 

 municates with the intestine. The digestive system is a very 

 simple one and will require little or no demonstration by the in- 

 structors. 



5. Cut the digestive tract near the posterior end and reflect it 

 forward. This exposes the cartilaginous endocramum. The 

 central nervous system lies directly beneath the endocranium, 

 which -must first be removed in order to expose the nerve ring. 

 This can best be dne by cutting along the median line of the car- 

 tilage with scissors, beginning at the anterior end and taking great 

 care not to damage the tissues beneath. Reflect the halves of the 

 endocranium laterally. This exposes the nerve ring and ventral 

 nerve cord. Carefully clear up the area ; work the nerves free 

 with fine curved forceps or needles. Take great care to damage 

 the nervous structures themselves as little as possible. Finally 

 work back along the ventral nerve cord and exposethe five ganglia ; 

 do not lift the cord as the nerves radiating from the ventral sur- 

 face of the ganglion will thus be completely destroyed. Special 



1 The circulatory system of young specimens (carapace of three inches) 

 may be injected through the heart by inserting the needle in the median line 

 in the groove between the carapace and abdomen. The entire circulatory 

 system may be followed by removing the dorsal part of the carapace, since the 

 injected vessels may be readily seen through the semi-transparent ventral body 

 wall. Larger specimens may be similarly injected with some advantage in 

 dissecting. 



