BALANOGLOSSUS. 253 



been determined. The primary anterior enterocoel in the 

 Echinoderm larva is not quite the same as the correspond- 

 ing cavity in Tornaria, since it contains also the elements 

 of the general body-cavity. Apart from slight differences, 

 the collar-cavities and general body-cavities arise essen- 

 tially in the same way in Tornaria as they do in the case 

 of the direct developing larva of Balanoglossus (see above).* 

 In the Echinoderm larva, however, the paired body- 

 cavities do not arise as independent archenteric pouches, 

 but they become constricted off from the anterior entero- 

 ccel. Making allowance for these deviations in the origin 

 of the body-cavities, --deviations which are by no means 

 fundamental, since in both cases the body-cavities are 

 ultimately reducible to archenteric pouches, --it is an 

 extremely striking fact that both in Tornaria and Auricu- 

 laria the anterior enterocoel acquires an opening to the 

 exterior on the dorsal surface to the left of the middle line. 

 This opening is called the water-pore, since it forms the 

 outlet (possibly both outlet and inlet) of the water-vascular 

 system of the Echinoderm. In Tornaria it persists after 

 the metamorphosis as the proboscis-pore, which has been 

 described above. 



The Larva of Asterias vnlgaris ; Water-pores and 

 Praoral Lobe. 



In view of what was said above as to the occurrence of 

 paired proboscis-pores in B. Knpfferi, it is interesting to 

 note that sometimes there are two water-pores, a right and 

 a left, in Echinoderm larvae. This has been observed by 



* As to the origin of the body-cavities in. different species of Balanoglos- 

 sus, MORGAN summarises his observations as follows : " They may arise as 

 enteric diverticula, as endodermal proliferations, or even arise from mesenchy- 

 matous beginnings." (See MORGAN. No. 125 bibliog.) 



