206 MORPHOLOGY OF INVERTEBRATE TYPES 



only with the dorsal lacuna, but also with its collateral lacuna 

 which runs along the intestine in the line of attachment of the 

 mesentery. 



The nervous system is composed of an ectoneural and a hypo- 

 neural system resembling each other and separated by an ex- 

 tremely fine membrane. The oral nerve ring is situated in the 

 peristome. It gives rise to five radial nerves running to the 

 cloaca. The ectoneural ring gives rise to ten tentacle nerves, 

 the ectoneural radial nerves to a subepithelial plexus. The 

 hyponeural or motor system supplies nerves to all the muscles. 

 An entoneural system has as yet not been described and is 

 probably wanting. 



Reproductive system. The sexes are separate, but the 

 anatomical structure of the reproductive organs is similar in 

 both sexes. There is but a single gonad composed of two clus- 

 ters of numerous tubes. In the case of the male, these tubes 

 constitute the testis and open into a small seminal reservoir. A 

 seminal duct or vas deferens runs forward from this reservoir and 

 terminates in an interradial genital opening at the base of the 

 tentacles in the mid-dorsal line. In the case of the female, the 

 tubes constitute the ovary and open into a common oviduct 

 which terminates in the genital opening. The latter has the 

 same situation as in the male. The sexual cells, copulation being 

 absent, are discharged into the water and fertilization left to 

 chance. Development is characterized by a metamorphosis 

 with a larval stage known as Aurkularia. 



Instructions 



i. Place the specimen in a dissecting tray with water. Find 

 the smallest pair of tentacles belonging to the middle radius 

 (fifth radius) of the trivium. Turn the specimen so that this 

 radius would be on your right. Make a drawing of the specimen 

 in this position (lateral view) showing tentacles, mouth, am- 

 bulacral tubes, and anus. 



