114 



COMPARATIVE ANATOMY 



CHAP. 



separately on the stem. These dislocated portions or organs are able 

 to multiply independently. 



II. The Diseonanthe (Disealia, Porpeta, Porpalia, Velella). 



These have to be interpreted quite differently from the Siphonanthe. 

 According to the harmonious and convincing teaching of ontogeny and 

 comparative anatomy, these animals must be considered as Medusce 

 with marginal tentacles. These Medusce have a gastric tube with 

 mouth (principal siphon) in the centre of the subumbrella in the 

 typical way, but also produce secondary siphons or palpons, by gemma- 

 tion, on the subumbrella (just as in the G-astroblasta) ; out of the 

 wall of these secondary siphons the Medusa-shaped gonophores 



FIG. 87. Porpalia prunella, after Haeckel. cd, Central gland ; Ik, air chamber ; cp, central 

 pore of the same ; rk, radial canal ; sp, supporting plate ; eu, exumbrella ; su, subumbrella ; t, ten- 

 tacles ; g, gonades ; o, mouth ; ms, principal siphon ; gm, accessory siphons. 



bud (Fig. 87). All the tentacles belong to the margin of the un- 

 divided persistent Medusa umbrella. In the umbrella (on which in 

 Velella a vertical crest, generally placed diagonally, rises) an air 

 vesicle is developed on the exumbrellar side ; this is often of very com- 

 plicated structure, many chambered, and originally octoradiate ; the 

 chambers communicate with the exterior through numerous pores. 

 The young stages of the Diseonanthe are typical Medusce, with 8 (later 

 16) tentacles at the disc margin, and with one central gastric peduncle 

 or siphon. The gonophores detach themselves as free -swimming 

 medusoid sexual persons, and only ripen the sexual products after their 

 separation. 



The view of the Siphonophoran body here brought forward takes up 

 a position intermediate between two diametrically opposed theories, 



