82 



BEIT ARO OOTO. 



of the axial sinus grow outwards (i.e. towards the periphery of the disc) in two 

 horns, which again grow in, each towards the next adjoining radius and there he- 

 come apposed to similar horns from the next interradii. These horns lying in 

 the radii unite with the other portions of the perihamial system, which have a 

 quite different origin. 



The portions of the perihremal system lying peripherally from whithin the first 

 pair of tube-feet arise in Aslerina gibhoxa in exactly the same way as the entire 

 system does in A .* (cria s pallida, viz. as separate solid masses of mesenchy matous 

 cells lying in pairs on the oral side of the radial water-vascular tubes, between the 

 successive pairs of tube-feet. These solid masses subsequently acquire lumen and 

 growing towards each other unite. The radial septum, i.e. the septum between 

 the members of each pair, is never absorbed. This would seem to reduce our 

 conception of the origin of the perihernial system to chaos, but in the face of ob- 

 served facts we should only wait for a higher generalisation to bring harmony 

 into the subject. It may be added that we have parallel cases in other groups, 

 where the same organ arises differently in different species (collar-cavity in En- 

 teropneusta, certain organs in the bud-development of ascidians). 



In conclusion I must refer to the so-called "dorsal sac. " MacBride claims 

 to have proved the origin of this organ from the right anterior enterocœl, and he 

 therefore regards it as the homologue of the hydrocœl on the left side and calls 

 it the " right hydrocœl. " To support his opinion he refers to certain abnormal 

 larva; in which there were pore-canals on both sides of the body, the one on the 

 right side opening into the supposed right hydrocœl. I have not had the good 

 fortune of coming across any similar abnormality ; but the observation of normal 

 larvae has led me to a very different conclusion, entirely confirmatory of my ob- 

 seivation on Asterias. 



The cavity in question arises in fact from the anterior end of the left post- 

 erior enterocœl. Its right end is closely apposed to the wall of the right posterior 

 enterocœl, as in Asteria*, but there is at no time any connection between the two. 

 Its origin from the left posterior enterocœl is, on the contrary, very distinct and 

 unmistakable. The series of sections that I shall reproduce in my full paper 

 will, I think, put the matter beyond doubt. MacB ride's idea of the homology 

 in question is thus entirely deprived of its ground. 



The differences above sketched between Asterias nallida and Aster ina gibbosa 



