378 MANUAL OF ZOOLOGY. 



oped in the interior of the polypide. The statoblasts are found in certain 

 seasons lying loose in the perigastric cavity. In form "they may be 

 generally described as lenticular bodies, varying, according to the species, 

 from an orbicular to an elongated-oval figure, and enclosed in a horny 

 shell, which consists of two concavo-convex discs united by their margins, 

 where they are further strengthened by a ring which runs round the entire 

 margin, and is of different structure from the discs. . . . When the stato- 

 blasts are placed under circumstances favouring their development, they 

 open by the separation from one another of the two faces, and there then 

 escapes from them a young Polyzoon, already in an advanced stage of de- 

 velopment, and in all essential respects resembling the adult individual in 

 whose cell the statoblasts were produced " (Allman). The statoblasts are 

 formed as buds upon the "funiculus" the cord already alluded to as ex- 

 tending from the testis to the stomach upon which they may usually be 

 seen in different stages of growth. They do not appear to be set free from 

 the perigastric space prior to the death of the adult, and when liberated 

 they are enabled to float near the surface of the water, in consequence of 

 the cells of the marginal ring, or " annulus," being spongy and filled with 

 air. They must be looked upon as " gemmce peculiarly encysted, and 

 destined to remain for a period in a quiescent or pupa-like state " (All- 

 man). 



As regards the development of the Polyzoa, the embryo 

 upon its emergence from the ovum presents itself as a ciliated, 

 free-swimming, sac-like body, from which the polypide is sub- 

 sequently produced by a process of gemmation. In the singu- 

 lar Rhabdopleura the primitive bud is enclosed between two 

 fleshy lobes or valve-like plates, attached along their dorsal 

 margin, and giving exit in front to the rudimentary lophophore. 

 As the development proceeds, these plates cease to keep pace 

 in their growth with the rest of the bud ; till ultimately they 

 appear as a peculiar shield-like organ on the haemal side of the 

 lophophore. These lobes have been compared by Dr Allman 

 with the mantle-lobes of the Lamellibranchiata ; and according 

 to the most recent researches, the whole shield-like organ is a 

 specially modified zooid, and in no way corresponds with the 

 " epistome " of the Pkylactolczmata. 



DIVISIONS OF THE POLYZOA. According to the classification 

 proposed by Nitsche, and now generally adopted, the Polyzoa 

 are divided into the two primary sections of the Entoprocta and 

 the Ectoprocta, to which a third must be added, in accordance 

 with the views of Dr Allman, under the name of Aspidophora, 

 for the reception of the anomalous Rhabdopleura. The follow- 

 ing are the principal groups of the Polyzoa : 



A. ECTOPROCTA. 



Mouth within the circle of tentacles ; anus dorsal and outside the ten- 

 tacular circle ; lophophore crescentic or circular. 



Sub-order I. Phylactolcemata. 

 ii ii II. Gymnolieinata. 



