HYDKOMEDUSAE. 735 



pores on the circular canal. The Aequoridae which have a large funnel-shaped stomach and 

 a closeable mouth may also have the radial canals functioning as digestive organs. The con- 

 traction of the stomach, when the mouth is closed, would drive water into the canal system 

 and expel the stale water through the pores on the circular canal. The hypothesis that the 

 radial canals function as the digestive organs would perhaps account for the large number 

 usually present in the Aequoridae, some of which are the largest Leptomedusae known. 



Order TRACHOMEDUSAE. 



Family Petasidae, Haeckel, 1877. 



Family Character. Trachomedusae with four radial canals, upon which are situated 

 cylindrical, globular, or papilliform gonads. Stomach without a peduncle. Either external 

 sensory clubs, or external or internal sensory vesicles. 



Haeckel (1879) divided the Petasidae into two subfamilies, namely : 



Petnachnidae, without blind centripetal canals. Genera : Petasus, Dipetasus, Petasata, 

 Petachnum, Aglaurupsis and Gossea. 



Olindiadae, with blind centripetal canals. Genus : Olindias. 



Since Haeckel published his System der Medusen three more genera must be added to 

 the family, namely, Gonionemus (placed by Haeckel among the Thaumantidae under a wrongly 

 spelt generic name Gonynema), Gonionernoides and Vallentinia. 



A more natural classification of the Petasidae can, I think, be obtained by taking the 

 structure of the sense organs instead of the centripetal canals for the characters of the two 

 subfamilies. 



There are two distinct types of sense organs : 



A. Sensory clubs (Horkolbchen) with a short stalk, which project from the margin of 

 the umbrella; with a single otolith and with external sensory hairs. These sensory clubs are 

 present in the genera Petasus, Diptasus, Petasata, Petnachnum ? (species not figured). 



B. Sensory vesicles (Horblaschen) situated either in the mesogloea (internal) or on the 

 margin of the umbrella (external) ; sessile and without external sensory hairs. These sensory 

 vesicles are present in the genera Aglauropsis, Gossea, Olindias, Gonionemus, Gonionernoides 

 and Vallentinia. 



On this classification the arrangement of the genera would be as follows : 



Subfamily Petachnidae, Haeckel, 1877. 



Petasidae with sensory clubs containing an otolith and with external sensory hairs. 



Genus Petasus. Species — P. atavus, P. tetranema. 



Genus Dipetasus. Species — D. digonimus. 



Genus Petasata. Species — P. eucope. 



Genus '>. Petachnum. Species — P. tiaropsis. 



All the above genera and species were described by Haeckel (1879), and I have not 

 succeeded in finding any notice of their being recorded by any other person. 



