SIGNIFICANCE OF CERTAIN LARV^ OF ECHINODERMS. 1/9 



The anterior pair extended into the preoral lobe where it may 

 have been united into a single cavity. Posteriorly its cavities 

 were placed on the right and left of the oesophagus and each 

 cavity opened to the exterior, on the dorsal surface of the animal, 

 through a ciliated duct. The posterior end of each anterior cavity 

 was connected with the corresponding cavity of the middle pair 

 by a second duct, also ciliated. The middle cavities were situated 

 on either side of the point of union of the oesophagus and stomach. 

 The posterior cavities were larger than those of the anterior and 

 middle pairs and were applied to the stomach, forming a mesentery 

 on the dorsal mid line. 



If Fig. II a, of this paper is compared with Bury's Fig. 45, 

 McBride's Fig. 157 and Bather's Fig. I it will be seen that the 

 general plan is the same with differences in detail only. 



Bury's idea that the hydroccele (left middle body cavity) encir- 

 cled the oesophagus (the right cavity having entirely disappeared) 

 even during the period of the free-swimming existence of the ani- 

 mal, is, in the light of recent observations, an unnecessary assump- 

 tion and one for which no explanation has been made. The 

 changes which take place in the posterior pair of body cavities of 

 echinoderm larvae, by which the left one becomes horseshoe-shaped 

 and encircles the stomach, are almost exactly similar to those by 

 which the left middle body cavity takes on the form of a ring sur- 

 rounding the oesophagus. If to explain the former it is neces- 

 sary, as Bury and others believe, to assume a shifting of the posi- 

 tion of the mouth and oesophagus incident to a life on the bottom, 

 then a similar explanation for the latter is also called for. I agree 

 with the more recent writers in the assumption that both the hy- 

 drocele and left posterior body cavity acquired their circular 

 shape and position around the alimentary canal, at the same 

 time, viz., during the period when the entire organization of the 

 animal was being readjusted to its new conditions of life on the 

 bottom. 



According to McBride's hypothesis, each of the middle body 

 cavities possessed, during the free swimming stage of the an- 

 cestor, five tentacles which were used in the capture of food. 

 There is good evidence for the existence of two hydrocoeles 

 (middle body cavities), as McBride has shown in his work on 



