174 INVERTEBRATE ANIMALS. 



open ocean and some exhibit the phenomenon of phospho- 

 rescence. 



In the foregoing description it has been found impossible to convey even 

 the most elementary outline of the anatomy of a Tunicate without having 

 recourse to technical terms. There still remain a few points of homology 

 which should be mentioned. In the foregoing, the so-called "oral aperture" 

 of the animal has been regarded as truly the mouth, this being the simplest 

 view, and the one held by Prof. Huxley. Upon this view the " respiratory 

 sac," into which the mouth opens, must be regarded as a greatly-developed 

 pharynx (i. e., the upper portion of the alimentary tube). Similarly, on this 

 view, the lower aperture of the respiratory sac will have to be regarded as 

 the opening of the gullet. By Prof. Allman, again, the respiratory sac is 

 looked upon as formed by a great modification of organs corresponding to 

 the ciliated tentacles of the Polyzoa, so that the lower aperture of the respira- 

 tory sac is the true mouth. Lastly, by Prof. Rolleston the respiratory sac is 

 looked upon as corresponding to the gills of the Bivalve shell-fish (Lamelli- 

 branchiatd), and the oral and atrial apertures are regarded as corresponding 

 to the " respiratory siphons " of these same animals. On this view, the 

 lower aperture of the respiratory sac is again looked upon as the true mouth. 

 The question cannot be regarded as settled, and Huxley's view has been 

 here adopted merely as being the most readily intelligible to learners. 



CLASS III. BEACHIOPODA. The members of this class are 

 little known to the general public, being all marine, often in- 

 habiting considerable depths in the sea, and being much more 

 abundantly represented by fossil forms than by living ex- 

 amples. They are often placed with the ordinary Bivalve shell- 

 fish (Lamellibranchiata], in consequence of their universally 

 possessing a shell composed of two pieces or valves (Fig. 79), 

 but they are really of a much lower organization. In their 

 essential structure they show many points of affinity to the 

 Polyzoa, but they are always simple animals, never forming 

 colonies, and they always have a bivalve shell. The two pieces 

 of which the shell is composed are always placed one in front 

 and one behind, so that they are "ventral" and "dorsal," and 

 not "right" and "left" as in the true Bivalves. The two 

 valves of the shell are also always slightly, and sometimes 

 greatly, different to one another in size, so that the shell is 

 said to be " inequi valve." The ventral valve is usually the 

 largest, and often possesses a prominent curved beak, which 

 is generally perforated by an aperture through which there 

 passes a muscular stalk by means of which the shell is at- 

 tached to some solid object. In some cases, however, as in 

 Lingula (Fig. 79), the stalk of attachment simply passes be- 

 tween the valves, and is not transmitted through a distinct 

 aperture. In other cases the shell is simply attached by the 

 substance of the ventral valve. 



