INVERTEBRATE ZOOLOGY. l3S 



3. Circulatory and respiratory systems. Heart, with one atrium and one 



ventricle, distingiiished by the thickness of their walls. The circu- 

 lation is jmrtly in blood vessels, and partly lacunar, there being no 

 capillaries. The blood from the coelom and other blood sinuses is 

 brought into the respiratory chamber by a ring-vessel, which runs 

 along the mantle-ridge and branches into a net-work of fine vessels. 

 These are re-collected into a pulmonary vein, which conveys the now 

 aerated blood to the atrium. A single aorta passes from the ventricle, 

 but soon gives off the visceral artery towards the apex of the visceral 

 sac. The aorta then passes into the body cavity. Its main branches 

 are intestinal, genital, salivary and jjedal, to the corresponding parts. 



4. Nervous system. The three typical ganglia, as seen in the clam, are 



here united into a complex lying upon the pharynx. The cerebral 

 ganglion is anterior, connected by a double cominissiare to the viscero- 

 2')edal ganglion just posterior to it. This latter is perforated by an 

 artery which divides it into an upper {=pedal) and a lower {=visce- 

 ral). Anterior to the cerebral is a small pair, attached by commis- 

 sures, the stomato-pedal. The following are the main nerves : — 

 From cerebral ganglion : tentacular (to ommatophores) outer-labial, 

 inner-labial, facial (to front of head and short tentacles) auditory (to 

 ear-sacs on side of pedal ganglion) a median nerve to the jjenis. 

 From viscero-iiedal : goiittd (to reproductive organs and heart), an- 

 terior, middle and posterior mantle nerves. A large number to foot, 

 mostly small. 



Type XXI. Bugula sp? 



I 06. Occurrence. Preservation. Bugula is a colonial polyp closely re- 

 sembling a hydroid, and found on all sorts of submerged and floating objects, 

 especially on eel-grass. Und(>r the lens a branch resolves itself into a colony 

 of sessile animals living in cells, into which they are capable of entirely with- 

 drawing. As the skeletal portion is very durable, a great many of the 

 branches collected will be found partially or wholly empty, and thus each 

 specimen must be examined before preserving. They should be treated as in 

 the case of hydroids in order to prevent the retraction of parts. 



I 07. Study of a .'staiiwd S2)eciinen. Select a good Ijranch fixed with parts 

 expanded, and stain and mount entire. Notice the separate cups or cells, 

 zooecia, in which the separate animals live. How are they arranged, rela- 

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