MOLLUSCOIDA. 193 



siJerable resemblance to the tadpole of a frog. The tail of the 

 larval Tunicate also possesses in its interior a singular axial rijd, 

 which presents a remarkable resemblance to the embryonic back- 

 bone or " notochor J " of Vertebrate animals. There are, further, 

 other points of likeness between the embryo Tunicates and the 

 Vertehrata ; and the Tunicates are, theiefore, sometimes regarded as 

 degenerate Vertebrate animals. 



The Tunicata are all marine, but ditfer a good deal from one 

 another in form. In the so-called " simple " Ascidians the animal 

 has the shape figured above (fig. 134, C'), and is fixed to some solid 

 object by one end of the test. In the " social " Ascidians the 

 organism consists of a number of zooids, produced by continuous 

 budding, and connected together by a common tube, through which 

 the blood circulates. In the so-called " compound " Ascidians (fig. 

 134, B) the tests are fused together into a conmion gelatinous mass, 

 in which the individuals are endjedded in groups. Some of the 

 Tunicata are oceanic — that is to say, are found floating or swimming 

 at the surface of the open ocean — and some exhibit the phenomenon 

 of phosphorescence. 



Class III. Brachiopoda. 



The members of this class, sometimes called " Lamp-shells," are 

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

 ing considerable depths in the sea, and being much more abundantly 

 represented by fossil forms than by living examples. They are 

 often placed with the ordinary Bivalve shell-fish (LameUibranchiata), 

 in consequence of their universally possessing a shell composed of 

 two pieces or valves (fig. 13r>), but they are really of a much lower 

 organisation. In their essential structure they show manj^ points 

 of affinity to the Piib/:o<i^ 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 from 

 one another in size, so that the shell is said to be " inequivalve." 

 The ventral valve is usually the largest, and often possesses a 

 prominent curved beak, which is generally perforated by an aper- 

 ture tlirough wdiich there passes a muscular stalk by means of 

 which the shell is attached to some solid object. In some cases, 

 however, as in Liiigula (fig. 136), the stalk of attachment simply 

 passes between the valves, and is not transmitted thi-ough a distinct 

 aperture. In other cases the shell is simply attached liy the sub- 

 stance of the ventral valve. 



