BIPENNARIA LARVAE OF ASTERINA (PATIRIA) MINIATA. 1 19 



larvae aged about three and a half days of a strictly bilaterally 

 symmetrical condition of the hydroccels, madreporic pores, and 

 pore-canals. Soon after this period the right madreporic pore 

 closes and the pore-canal, without an external opening, persists 

 for a short time and then entirely disappears. 



Several continental writers had observed the occasional occur- 

 rence in asteroid larvae of paired madreporic pores, but had con- 

 sidered the condition as a pathological one. Field, however, 

 maintains that there normally occurs, in Asterias vulgaris at least, 

 a transitory stage in which bilateral madreporic pores exist, and 

 considers this condition as " a true ontogenetic character, and of 

 very considerable phylogentic significance." Gemmill (1912) was 

 able partially to confirm Field's observations of the occurrence of 

 paired water pores in the genus Asterias, finding this condition in 

 fifty per cent, of larvae of Asterias glacialis and in about ten per 

 cent, of those Asterias rubcns. He also states that in all cases 

 the water-pore soon closes. 



Paired echinus-organs, madreporic vesicles, and other deriva- 

 tives of the right hydroccel, were observed by MacBride (1911) 

 in two very advanced Plutei, one of Echinus millaris and the other 

 of Echinus esculentis. A similar condition, was observed by 

 Grave (1911) in a single Pluteus of the sand-dollar Mcllita penta- 

 pora, though the figure shows the two madreporic canals making 

 exit through a single median dorsal madreporic pore. 



PAIRED MADREPORIC PORES IN ASTERINA LARVAE. 



During the months of April, May and June, 1920, I had occa- 

 sion to observe the development of a very large number of cul- 

 tures of larvae of Asterina (Patina) miniata at Pacific Grove, 

 California. In one culture consisting of otherwise normal, 

 healthy larvae three weeks old, I noticed a larva with two perfect 

 madreporic pores and pore-canals. Further search, involving a 

 complete census of all the larvae in the culture, revealed twenty- 

 six more larvae with right madreporic canals in some cases as well 

 developed as the left ; but in others with the right pore closed and 

 the pore-canal smaller than the one on the left. More than half 

 of all the advanced larvae in this culture had the double madre- 



