780 THE ANIMAL KINGDOM. 



Halistemma tergestinum, Claus, Arb. Zool. Inst. Wien, i, 1878; Agalmopsis 

 utricularia, Id. op. cit. ii. 1879; Porpitidae and Velellidae, Agassiz, Mem. Harvard 

 Mus. viii. 1881-84; Velella and Rataria, Bedot, Recueil Zool. Suisse, i. 1884; the 

 same and Porpita, Id. op. eit. ii. 1885 ; Notes on Rhizophysa and Physalia, Chun, 

 SB. Akad. Berlin, 1882, pp. 1168-70; Diphyes subtilis, Id. op. cit. 1886, p. 681 ; 

 Diphyozooids, Id. ibid. p. 686; Cyclical evolution and Monophyidae, see p. 774, ante. 



Histology in general, Korotneff, Mitth. Zool. Stat. Naples, v. 1884. Nervous 

 system of Porpita, Conn and Beyer, Studies from Biol. Laboratory, Johns Hopkins 

 Univ. iii. 1883 ; of Velella, Chun, Z. A. iv. 1881. 



Origin of sexual cells. Weismann, Entstehung der Sexualzellen bei den Hydro- 

 medusen, Jena, 1883, pp. 193-213; pp. 212, 265, 291. 



Development. Entwickelungsgeschichte der Siphonophoren, Haeckel, Utrecht, 

 1869 ; Metschnikoff, Z. W. Z. xxiv. 1874 ; ofAgalma, Fewkes, Bull. Mus. Harvard, 

 xi. 1885. 



SUB-CLASS 2. ACRASPEDA *. 

 (Acalephae, Phanerocarpa, Steganopthalmata, Scyphomedusae). 



Hydroid form known in a few instances only ; small and fixed, protected 

 partially in some instances by a perisarcal tube ; with a peristomial disc 

 bordered by sixteen (or more) solid tentacles; mouth squarish and gastric 

 cavity traversed by four vertical interradial ridges ; multiplying either by 

 lateral buds which are detached, or by creeping stolons which throw up buds 

 at intervals. The Medusa is derived from it by multiple transverse fission 

 of the oral extremity of the body, giving rise to a temporary form known as 

 Strobila. 



The Medusa has a square-shaped manubrium, the oral angles of which 

 are sometimes prodticed into four simple or branched arms ; bifid marginal 

 lobes to the bell rarely absent, either four perradial or four perradial and four 

 interradial, supporting as many sensory bodies, i.e. tentactdocysts, or r hop alia ; 

 more or fewer gastral filaments disposed in rows or groups (pkacelli) within 

 the central gastric cavity at the sides of the manubrium ; and genital organs 

 similarly placed with a genital epitheliitm of endodermal origin. The sexes 

 are separate: development is direct in Pelagia. There is no velum to the 

 bell. Tentacles may be absent, of, if present, either situated on the exumbrella 

 or subumbrella. The nervous centres are contained in the marginal sensory 

 bodies. Marine. 



The non-sexual Hydroid form of the Acraspeda, the Scyphostoma or 

 Hydra tuba, is only known in genera of the Ephyroniae or second division 

 of the sub-class 2 . It is relatively small in size (half-an-inch or less), and its 



1 See p. 747, ante, for Gb'tte's view as to the position of this sub-class. 



8 That is to say in Nausithoe marginata, Chrysaora (C. isosceles, C. Mediterraned), Cyanea 

 (C. arctica, C. capillata, C. Annaskala), Aurelia (A. aurita, A.jlavidula), Pilema pulmo = Rhizo- 



