HYDROZOA ACRASPEDA. 781 



special characters are as follows. The mouth is dilatable and somewhat 

 squarish in outline, especially in early stages : its margin is thickened and 

 armed with cnidoblasts : it is situate in the centre of a disc or peristome of 

 great mobility. The margin of the peristome carries normally sixteen 

 tentacles : the number may however be greater, e. g. twenty or thirty-two. 

 The tentacles are contractile, and have a solid core of endoderm cells. 

 Four of them, the perradial tentacles, which are first developed, correspond 

 to the four angles of the mouth ; four others, the interradial tentacles, 

 second in development, to the centres of the square sides of the mouth, and 

 the remaining eight adradial tentacles occupy the intervals between the 

 per- and inter-radial. The body is somewhat elongated, but contracts 

 below into a peduncle, by which the animal is affixed to some foreign object 

 (stones, sea-weeds, &c.). This peduncle is slender in Chrysaora and Cyanea, 

 and inclosed in a gelatinoid sheath secreted by the ectoderm 1 . The young 

 Nausithoe marginata inhabits a perisarcal tube. The internal aspect of the 

 gastric cavity is traversed by four vertical ridges, septa or taeniolae, which 

 correspond one to each of the four interradial tentacles. They are project- 

 ing ridges of mesoglaea covered by endoderm cells and inclosing a tube of 

 muscle cells of endodermal (Claus) or ectodermal and peristomial (Gotte) 

 origin. They have been compared to the mesenteries of Anthozoa. The 

 tentacles especially are provided with nematocysts of two sizes, and the 

 smaller (microcnidae) have extremely long cnidocils. Cilia occur upon the 

 tentacles, in greatest numbers near their bases, as well as upon the peristome 

 and margins of the body. The Scyphostoma multiplies in two ways, by 

 means of lateral buds which are detached, and by two to three creeping 

 basal stolons, from which buds are thrown up at intervals 2 . 



stoma Cuvieri, Pseudorhiza aurosa, Stylorhiza punctata. As these genera represent several families, 

 there can be little doubt that the Scyphostoma is of general occurrence. 



1 L. Agassiz states that the sheath is sometimes wanting in Cyanea, and the young animal may 

 be seen creeping on its tentacles. 



2 F. E. Schulze has described (A. M. A. xiii. 1877), under the name Spongicola fi stularis, a 

 Hydroid which he observed inhabiting the canal-system of several marine sponges, e. g. Reniera 

 fibulata, Esperia Bauriana, &c. It forms a colony of nearly vertical branching tubes invested by a . 

 chitinoid perisarc. The Hydroid itself has the typical structure of the Acalephan Hydroid as given 

 above. Its peristome measures about i-i| mm. in diameter. It has 16, 20, &c. up to 40 solid 

 contractile tentacles, and the taeniolae extend across the peristome to the margin of the mouth. The 

 peristome is bordered by a strong circular fold. The exposed part of the body and the tentacles are 

 ciliated. The fore-part of the body bearing the tentacles can be invaginated. Allman has also 

 described a sponge-inhabiting Hydroid, Stephanoscyphus mirabilis, which closely resembles 

 Spongicola. It is colonial, has a perisarcal tube, &c. But its great peculiarity, according to its 

 discoverer, is the presence of a circular and four longitudinal canals. It is probable that the 

 structures in question are really four taeniolae and a circular fold such as are described above. See 

 Allman, Tr. L. S. (2), i. 1879. Schulze's Hydroid is probably identical with the structures described 

 by Eimer (Tageblatt der Natf. Versaml. in Leipzig, 1872, p. 62), and supposed by him to be integral 

 parts of the sponge in which they occur. 



Kowalewsky, quoted by Metschnikoff (Embryol. Studien an Medusen, 1886, p. 88) appears to 

 have seen the strobila of Spongicola and the detachment of Ephyrae. 



