OBSERVATIONS ON POLYZOA. 217 



and Cyclostomata and the Phylactolsemata, show that 

 throughout all the modifications of succeeding and more 

 complicated genera, the class as a whole retains the nem'al 

 system at the posterior pole. 



Although cephalization, as shown by Mr. Morse, is 

 the tendency of the Saccata as a whole, still it must be 

 admitted, that it is not the tendency of the lowest class, 

 but, on the contrary, this class betrays throughout an 

 anti-cephalic character. 



From the intensely cephalized Urnatella and Pedicel- 

 lina we have no cephalized forms until we reach Pedicel- 

 lina. This, with its two invaginated folds, is a more 

 cephalized type than Fredericella, which in its turn is 

 more invaginated, and therefore more cephalized than the 

 higher forms of Plumatella, such as P. vitrea. Lophopus 

 and Pectinatella lead us gradually to Cristatella, which is 

 as completely evaginated as any of the Ctenostomata or 

 Cyclostomata. Thus, it is clearly shown that the clifier- 

 entiation of the three regions of the body by the elevation 

 of the neural region to the posterior pole is the govern- 

 ing tendency of the organic changes of the class. 



DESOEIPTION or SPECIES. 



The measurements were computed by a micrometer set 

 in Tolles' B orthoscopic eye-piece with a 2 inch objective, 

 and are given in decimal parts of a "millimetre," and also 

 in equal numbers. The equal numbers roughly computed 

 are each equivalent to yV m. m. or f^g inch, according as 

 they are measured upon the French scale, or an English 

 steel ruler. The statoblasts are drawn upon the enlarged 

 scale of aV m. m. or -ji-w i^ch to yg of an inch, and conse- 

 quently the figures give the relative sizes. The annuli were 

 measured on either side, and the results are contrasted by 

 placing the widths on the upper and lower side of a di- 

 viding line. The figures of the statoblasts being drawn 

 upon the same scale, the difierent varieties of the same 



COMMUNICATIONS ESSEX INSTITUTE, VOL. V. 29 MAKCII, 1868. 



