138 INVERTEBRATE ZOOLOGY. 



I I 8. Study of larvae. These are found in tow and are to be treated as 

 in 114. The larva is termed the Pluteus, similar to that of Ophiuridea, biit 

 distingnished from that of other Echinoderms by the great length of the lat- 

 eral arms. 



Type XXIV.— Balanoglossus sp? 



I I 9. Occurrence. This is one of the niimeroiis forms of mnd-worms or 

 clam-worms, foiind in miid-flats at low tide. It formerly received but little 

 attention, but has lately become an important form, on account of its proba- 

 ble kinship to Tunicata. and hence its relationship to Vertebrates. According 

 to some it possesses also affinity to the Echinoderms. Some species develop 

 directly, others hy a free-swimming pelagic larva, the Tornaria. similar to 

 the larvae of Echinoderms. 



I 20. External anatomy. The body is bilateral and divided into three 

 regions, proboscis, collar and body proper. The first two are rounded, the 

 body somewhat depressed. The dorscd side is marked by the presence of 

 paired gill-slits, which lead from the alimentary canal, and open dorsally by a 

 series of pores. A median stripe upon both dorsal and ventral siTrf aces marks 

 the position of median blood-vessels and median nerve cords. The mouth leads 

 in from the anterior edge of the collar iipon the ventral side. The anus is 

 situated at the posterior extremitj'. 



121. Internal anatomy. Owing to the small size of the animal, and the 

 partial filling up of the coelom with muscular fibres and connective tissue, 

 simple dissection is impracticable. The parts and their relations must be 

 sttidied by sections, the preparation of which is rendered difficult by the pecu- 

 liar consistency of the animal. Treated in the usual way, the tissues are apt 

 to harden to the consistence of horn, and become impossible to cut. Better 

 results may be obtained by imbedding in celloidin. The most useful sections 

 are cross-sections through the lower portion of the jiroboscis. the collar and 

 the branchial region of the body : and a median longitudinal section cut 

 dorso-ventrally through the anterior portion. The following synopsis of the 

 systems may be helpful : 



1. Digestive system. A nearly straight canal, running through the entire 

 body, from the ventrally situated moidh at anterior edge of collar, to the ter- 

 minal anns. Of morphological importance is an anteriorly extended dorsal 

 diverticulum in the region of the collar, considered by some the homologue of 



