4 6 THE ANTHOZOA 



longer possible to keep the two groups apart in a scheme of 

 natural classification. They must be considered as belonging to 

 an order Actiniidea, and as belonging to the same line of descent 

 from a common Edwardsia-like ancestor. The structure of the 

 corals will be detailed further on. Besides the biradial six-rayed 

 Actinians there are forms which, in external characters, bear the 

 closest resemblance to the ordinary sea-anemones. The resem- 

 blance extends to their histological characters, yet they differ 

 considerably in the number and arrangement of their mesenteries. 

 There is the family of Tealiidae, containing sea-anemones undis- 

 tinguishable from others in external appearance. Tealia crassicornis 

 and T. tuberculata are common on the British coasts. In these the 

 tentacles and mesenteries are arranged not in multiples of six but 

 of five. In T. crassicornis there are ten couples of complete mesen- 

 teries of equal size, two couples of which are directives. Between 

 these are ten couples of smaller mesenteries, and again in the 

 exocoeles between the first and second cycles twenty couples of still 

 smaller mesenteries (see Fig. XXII. 1). 



It seems difficult to connect this arrangement with the six- 

 rayed type, but the following ingenious suggestion is given by 

 Boveri : The complete mesenteries numbered 1 correspond to the 

 six couples of the first cycle in Actinia. Those numbered l a , the four 

 couples which are added to the other six to make up the apparent 

 first cycle of ten, belong in reality to the second cycle, but are 

 precociously developed and intruded amongst the first cycle. The 

 two couples of mesenteries numbered 2 are the remaining members 

 of the second cycle, and to them are joined the eight couples of 

 mesenteries numbered 2 a , precocious members of the real third 

 cycle, which, when added to the two couples 2 a , make up the ten 

 couples of the apparent second cycle. And so on for the remaining 

 cycles. 



Boveri's suggestion is not only very ingenious, but is sup- 

 ported by a peculiar sequence of mesenterial development observed 

 in an undetermined larva which he suspected to be that of a 

 Tealia. The reader is referred to his memoir (10) for details. 

 Accepting his suggestion, we may provisionally consider the 

 Tealiidae as an offshoot of the six-rayed Actinians. 



Polyopis striata has been described by K. Hertwig. It is a 

 small Actinian from the Challenger Collection, with thirty-six ten- 

 tacles reduced to stomidia, and is described as having eighteen 

 couples of mesenteries six couples complete, of which two couples 

 are directives, and in each of the sulco-lateral and sulculo-lateral 

 chambers three couples of incomplete mesenteries, the middle couple 

 being the longest (Fig. XXII. 2). According to this descrip- 

 tion we may, with Boveri, derive Polyopis from the normal 

 biradial type by suppression of the mesenteries in the lateral 



