HYDROZOA ACRASPEDA., 787 



developed in most Stauro- and Peromedtisae, extending along the ex- 

 umbrellar wall of the gastric cavity to a greater or less extent. The funnel 

 cavities penetrate them in some Lucernaridae and the Peromedusae. The 

 gastral filaments may be only four, as in Tessera, but are generally, especially 

 in the Peromedusae, extremely numerous, and in Cttbomedusae aggregated 

 into a single or double group at the axial ends of the gastric septa. In 

 Stauro- and Pero-medusae they frequently extend along the exumbrellar 

 wall of the gastric cavity even to its centre. The genital organs are 

 situated on the subumbrellar walls, but in one section of Lucernaridae, 

 the Halicyathidae or Cleistocarpidae, are contained in special mesogonial or 

 gastrogenital pouches opening into the central gastric cavity and placed 

 perradially in the cavity of the bell. The organs are either horseshoe 

 shaped, the convexity of the curve being adcentral, or the two limbs of the 

 horseshoe are separate (Lucernaridae, Peromedusae). In the Cubomedusae 

 there are eight genital lamellae, attached two to each gastral septum, one 

 on either side 1 . 



The development of the Tesseroniae is unknown save to a certain 

 extent in a Lucernaria. In it segmentation is equal and results in the 

 formation of a morula. The endoderm is formed either by delamination 

 from the ectoderm cells at one pole or by an immigration (?) of ectoderm 

 cells at all points into the centre of the morula (Gotte). The larva is non- 

 ciliated (? in all cases) and creeps about ; it is elongated ; one pole is beset 

 with cnidoblasts, by the other it fixes itself 2 . The ovum of Ephyroniae is 

 either set free from the ovary and has then a vitelline membrane, or in 

 Nausithoe marginata a mucous coat with cnidoblasts, or, as in Chrysaora^ 

 remains enclosed in an envelope of follicle cells. In the first case it may 

 pass into the planula condition while attached to the oral arms of the female 

 (Aurelia, Cassiopeia), and it may even nearly attain the Scyphostoma stage 

 in this position (Cyanea Annaskala, Stylorhizapunctata), or the embryoes are 

 contained in pouches of the radial canals (Pseudorhiza aurosd). Segmenta- 

 tion is regular : the blastocoele is large in Pelagia and Chrysaora, small in 

 other cases. The endoderm is formed by an invagination the cavity of 

 which is linear : the gastrula mouth closes but the pole which corresponds 

 to it is denoted by the development of cnidoblasts, and is the one at which 

 the mouth of the adult is formed later on. The planula ( = two-layered 

 blastula) is sometimes ciliated and free-swimming (A.^lrel^a, Chrysaora). 

 When it fixes itself the mouth is formed : two perradial tentacles first 



1 On the nature of these genital lamellae, see O. and R. Hertwig, J. Z. xiii. p. 599 ; Claus, Arb. 

 Zool. Inst. Wien, i. 1878, p. 269 et seqq., and his Untersuchungen, p. 37. Their accounts 

 differ. 



2 Haeckel believes that the Tesseroniae possess a Scyphostoma stage ; Claus that they develope 

 direct. The former is supported by Haacke, who has observed a young Charybdaea Rastonii in 

 which a canal was prolonged aborally from the gastric cavity and closed at the apex of the ex- 

 umbrella by a thin lamella. See Z. A. ix. 1886. 



3 E 



