INVERTEBRATE ZOOLOGY. 139 



the notochord of the Chordata. A skeletal body, probablj- secreted by this 

 last, lies between it and the dorsal-wall of the alimentary canal, and sends 

 two processes around the canal itself. This is important as showing a skele- 

 togenotis quality in the diverticnhim. 



2. Circulatory system. Two median vessels, dorsal and ventral, which 

 give off lateral twigs. At the anterior end of the dorsal vessel, lying upon 

 the diverticulum, the vessel is enlarged, forming what Kohler and Bateson 

 call the " heart." 



3. Respiratory system. The "gills" consist of paired diverticula of the 

 dorsal-wall of the intestine opening e.rieimally through pores in the dorsal 

 integument. The blood passes to these either through the lateral branches 

 above given, or from a pair of lateral vessels (Kowalevsky). 



4. Reproductive system. Bisexual : the two sorts of glands closely resem- 

 bling each other. They consist of paired glandular pockets opening separately 

 by pores on the dorsal side. Some of these lie in the gill-region, but are gen- 

 erally best developed in the region just posterior to this. 



5. Excretory system. This function has been ascribed by Batesox to the 

 '?(;/.s-a/7>o;-^, a minute tube passing into the proboscis fi-om a median dorsal 

 opening just beneath the edge of the collar. According to others (Spexgel). 

 the dorsal pore is a means by which water passes into the proboscis, making 

 the latter an ambulacral organ . 



6. Nervous system. The central organ is a thick cord hing dorsally in the 

 collar region, from which extends a mid-dorsal cord through the body, accom- 

 panying the dorsal blood-vessel. Immediately behind the collar the dorsal 

 cord gives off two- lateral commissures, which unite ventrally in the region of 

 the first pair of gill-diverticula. and form a mid-ventral cord, which follows 

 the blood-vessel of the same name. 



I 22. Study of Tornaria larva. Certain species of Balanoglossus possess 

 a free-swimming larva, the Tornaria, occurring in oceanic tow collected near 

 the regions inhabited by the adult. They are studied and prepared in the 

 same manner as other minute larvae. 



