188 PROCEEDINGS OF THE AMERICAN ACADEMY 



eluded under Polypi what are now known as Hydroids and 

 Bryozoa. Milne-Edwards has demonstrated that the latter are 

 not Polypi, — their structure not being truly radiate, — but the 

 lowest order of Mollusks, and he called them Molluscoids. 

 Polypi and Hydroids, however, are still grouped together. 

 By Ehrenberg these are called Anthozoa, — which he divides, 

 further, into two groups : Zoocoralia those which are free, and 

 Phytocoralia, those which are attached ; but mider these groups 

 he has made a very unnatural distribution of the families, as 

 the young and adult of the same species may difier in this re- 

 spect, the young being sometimes attached, when the adult 

 is free. Professor Agassiz exhibited in illustration a speci- 

 men of Manicina areolata from Florida, the young of which 

 are sessile, whilst the adults are free. Milne-Edwards sub- 

 divided Polypi into Actinoids, Alcyonoids, and Sertularians, 

 which he considers as coequal groups, a division chiefly based 

 on the character of the tentacles and calycle ; but Professor 

 J. D. Dana has at last shown that the first two form one natu- 

 ral group, and the Sertularians another, thus for the first time 

 uniting the types of the class of Polyps together into one di- 

 vision. Professor Agassiz is however of opinion, that the 

 Hydroids should be removed from the class of Polypi, and 

 referred to that of Acalephas. They are pedunculated Me- 

 dusse in the same sense that Crinoids are pedunculated Aste- 

 roids. The true Polypi are divided by Dana into two orders, 

 the Actinarians and Alcyonoids. Professor Agassiz thinks he 

 has detected indications of superiority and inferiority of struc- 

 ture between these orders, founded on the structure and num- 

 ber of the tentacles, &c. Thus in Alcyonoids these are fringed 

 and definite in number and position, being two in the long 

 axis, and in three pairs on the sides, while in Actinoids they 

 are simple, and there is not the same regularity of number and 

 position. The former should therefore be regarded as rank- 

 ing higher than the latter. Among Actinoid Polyps some are 

 simple, while others are compound individuals. The former 

 would at first seem to stand highest in the scale, whereas they 



