THE LAEVA OF ASTEEIAS VDLGARIS. Ill 



p. 411). The condition above described^ having two bilate- 

 rally symmetrical water-pores, is found in larvse three and 

 a half to four and a half days old (PI. XIV, figs. 14 and 23). 

 In a short time (eight to twelve hours after formation of 

 the pores) the ectoderm pushes together, closing the external 

 opening of the right water-tube; but the rest of the tube 

 persists for some time, retaining its characteristic appear- 

 ance. Fig. 19 is one of a series of sections of the larva at 

 the stage after the closure of the right water-pore, but with 

 the pore canal still present. In the sections succeeding the 

 one here figured the water-tube of the right side [w. t.) was 

 seen to end blindly. These observations were all made upon 

 the living larvae and confirmed by sections. 



The presence of a right and a left water-pore has been 

 noticed by several Continental investigators, but has been by 

 them set aside as pathological. There is reason to believe, 

 however, that this is a true ontogenetic character, and of very 

 considerable phylogenetic significance. In figs. 21 and 22 I 

 have drawn two sections of a series made by Professor Brooks, 

 showing the bilaterally symmetrical water-pores and pore 

 canals found by him in several larvae of the same stage as that 

 figured in side view in fig. 14, and in dorsal view in fig. 23. The 

 specimens shown in section (figs. 21 and 22) were taken in 

 the surface collections at Wood's Hall. The sections are cut 

 in a plane nearly parallel with the dorsal surface of the larva. 

 In fig. 22 the section is seen to pass through the postoral 

 and the preoral regions. In the postoral part are seen the two 

 water-tubes cut transversely [w. t.). In fig. 21 only the post- 

 oral part is figured. The section passes through the water- 

 pores {w.p.). Figs. 14 and 23 are drawn from living speci- 

 mens obtained by artificial fertilisation. Specimens with two 

 water-pores can be found in considerable numbers among 

 larvae three and a half to four and a half days old ; but 

 normally two water-pores are not present in larvae after that 

 age. In examining a number of larvae of that age we could 

 not expect to find a very large percentage with two water- 

 pores, firstly on account of the individual variation in rate of 



