456 DR. J. F. GEMMILL ON THE DEVELOPMENT OF 



Next appears a pair of mesenteries in each lateral exoccele (No. 7 

 in text-figs.), and a little later another jiair (No. 8 in text-figs.) in 

 each ventral exoccele. These four new pairs, though they belong 

 to what may be called the 2nd developmental cycle, grow rapidly 

 aad take rank Avith the six primary pairs, the result being that 

 ten pairs of mesenteries of the 1st Order are produced. Mean- 

 time a pair of additional mesenteries has begun to form in each 

 dorsal exocoele (No. 9 in text-figs.). These are really the last of 

 the 2nd developmental cycle, and their growth is slow. 3rd cycle 

 mesenteric pairs (Nos. 10, 1 1 in text-fig. 5) next form in the remain- 

 ing 8 exocoeles and growing relatively quickly, take rank with the 

 two pairs last named as mesenteries of the Ilnd Order. There 

 are now 20 exocoeles, and in each of these a new mesenteric pair 

 (Nos. 12-16 in text-fig. 5) appears producing the 20 pairs of the 

 Ilird Order. It will be seen that, properly speaking, four of 

 these pairs (Nos. 12-13) belong to the 3rd cycle, and the 

 remainder to the 4th cycle. There are now 40 exoccieles and in 

 each of these a new mesenteric pair appears, 8 of these pairs 

 belonging to the 4th and the remainder to the 5th develop- 

 mental cycle, the whole forming the mesenteiies of the I\^th 

 Order. 



In the text-figs, appended, the first twelve mesenteries and the 

 succeeding mesenteric pairs are numbered in the order of their 

 formation. It will be seen that the mesenteries of the Ilnd, 

 Ilird, and lYth Orders tend to appear earliest in the dorsally 

 placed exocoeles. This is natural since the mesenteries of these 

 exocoeles belong to earlier cycles than those which apparently 

 correspond with them in the other exocceles. 



Boveri (2. p. 496) conjectured that the decamerous symmetry 

 of Tealia Sivose by the formation of an additional pair of 1st Order 

 mesenteries in each dorsal and ventral primary exocoele. He 

 based this conjecture on sections of an undetermined larva 

 obtained from the Naples Station. Faurot (5) states definitely 

 that in Tealia felina Linn. (^ Tealia crassicornis Gosse, i. e., the 

 shore Urticina) the four additional 1st Order mesenteric pairs 

 arise in the dorsal and lateral primary exocoeles. His description 

 and figures bear out this statement fully, but it is to be noted tha,t 

 the youngest specimen he examined had already 28 tentacles. 

 Assuming him to be right, then, in spite of superficial resem- 

 blances, there is a deep-seated developmental diflerence between 

 Urticina coriacea and Urticina crassicornis which will certainly 

 make them take rank as distinct species. Material is being 

 collected for a revisal of his work on this point. 



References. 



Appellof, a. — "Studien iiber Aktinien Entwickelung." 

 Bergens Museum, Aarbog, 1901, pp. 1-99, 



