26 



DISCOPHOR^. 



Part III. 



tentacles, three in number, are in different states of development; one is as fully 

 groAvn as the other tentacles, but the other two have only one prong, the other 

 one being in an incipient state and budding from the base of the other. In 

 regard to the second and third forms, we cannot say whether the prongs grow 

 simultaneously or one from the other; but most probably in both ways. 



Rarely there are instances of double-headed embryos (PI. X. Figs. 37 and 37'') ; 

 the one we have illustrated had a digestive cavity common to both heads, and 

 eight tentacles on one head, but only four on the other. We know nothing of 

 the mode of development of this anomaly. 



The walls of the body have about the same proportionate thickness as in the 

 last stage ; but there are some new" features to be noticed here. The bases of 

 four alternate tentacles are jorolonged inwardly so as to project, like triangular 

 buttresses (PI. X^ Fig. 5 P), into the digestive cavity. The breadth of these 

 projections nearly equals that of the base of the tentacles, and they do not extend 

 downwards along the wall of the body much farther than they do laterally, or along 

 the wall of the disk within the circle of the tentacles. They have no relation 

 whatever with the outside of the embryo, but are altogether made up of cells which 

 developed from the inner wall [h). The position of these projections con^esponds 

 to the base of the first four tentacles.^ 



^ SiEBOLD was the first to discover tliese pro- 

 jections, which he calls longitudinal swellings (Liings- 

 wiilste), and also points out their relation to the 

 first four tentacles. He says that they extend from 

 the base of the tentacles along the wall of the 

 digestive cavity to its very bottom. See his Beitriige 

 zur Naturgeschichte der Wirbellosen Thiere, Ueber 

 Medusa aurita, Danzig, 1839. — Sars (Wiegmann's 

 Arcliiv, 1841, vol. 1, pp. 24 and 25, PI. I. Figs. 

 31, 32, and 33) also calls these projections swell- 

 ings. He observed them in the seyphostoma of 

 Aurelia and Cyanea, and agrees with Siebold in 

 regard to their extent, and also their relation to 

 the tentacles. — Steenstrup, in his remarkable little 

 work upon alternate generation (" Ueber den Gene- 

 rationswechsel," Copenhagen, 1842, pp. 14 and 15, 

 PL I. Figs. 35-40), describes an animal which he 

 identifies with the seyphostoma of Cyanea, figured 

 by Sars (Wiegmann's Ai-chiv, 1841, etc.) ; but he 

 goes on to point out certain organs, which he 

 previously intimates the latter had overlooked. 

 " Von inneren Organen hat Sars nur vier rundliche 



erhabene LiingswUlste beobachtet," page 14. These 

 organs are, a circular canal which runs along the 

 circle of tentacles, and four other canals running 

 from this, at equal distances apart, to the edge of 

 the aperture in the annular meml)rane which stretches 

 across the mouth of the bell-shaped disk, and there 

 they join another circular vessel. The four canals 

 which run along the inner surface of the bell, from 

 the apex to the circular canal at the base of the 

 tentacles, he considers to be the same as the four 

 longitudinal ridges in the digestive cavity of Sars's 

 seyphostoma. But when we examine the figures 

 of Steenstrup we are struck with their remarkable 

 resemblance to certain naked-eyed Medusaj, espec- 

 ially Turris. In Fig. 40 we see the pendent pro- 

 boscis from which the four radiating canals take 

 their rise and pass down the inner surface of the 

 bell to the circular canal at the base of the tenta- 

 cles. As to the four canals, which, Steenstrup says, 

 run to the aperture of the annular membrane, and 

 the circular ring which they empty into, we feel 

 quite positive that they are nothing but the dupli- 



