528 BIOLOGY, 



the body, and is thus in the first act a venous or right 

 heart, in the latter an arteriose or left; consequently, 

 according to function this single pouch is compounded or 

 made up of the two hearts. 



The branchiae deviate entirely from those of the 

 Mussels and Snails, and exhibit a very complicated struc- 

 ture ; in the mussel-like families they present the form 

 of a trellis-shaped sac, as in the Ascidise, or that of fili- 

 form appendages to the feet, as in the Cirripedia, or they 

 are funiform as in the Brachiopoda. In the Pteropoda 

 they are very varied ; in the snail-like Heteropoda they 

 are mostly pectiniform or tuft-shaped ; in the Sepiae or 

 Cephalopoda phylliform or fin-shaped. 



The external character common to the whole class is 

 the cylindrical body, unto which may be aptly conjoined 

 the possession of special organs of motion; whether 

 such organs be prehensile in function, media of support, 

 or veritable instruments of progression. 



The snail-like Pteropoda, Heteropoda, and Cepha- 

 lopoda, have a head, but this is wanting in the mussel- 

 like Salpas, Ascidiae, Cirripedia, and Brachiopoda. 



THIRD CIRCLE. RESPIRATORY, CUTANEOUS ANIMALS. 



3228. Respiratory animals are Dermatozoa with a 

 predominating system of respiration. 



Now, the respiratory system is the tegument, which here 

 then must attain the highest pitch of perfection. 



This again takes place through the increased process of 

 oxydation, which produces induration of the parts. 



The vessels, which surround the tegument, must 

 nourish this portion of it to a greater degree and render 

 it harder or more compact than any other, whereby 

 alternating expansions and contractions thereof originate, 

 in other words, the structure of the trachea. 



3229. The whole body of the Respiratory animals 

 becomes a trachea, a series of rings. The Respiratory 

 are therefore the Annulate animals. 



The rings are to be regarded as cysts, which are 

 mutually juxtaposited or repeated, not being inserted 



