Mollusca. qg 



Tunicata. 



The visitor looking into tank Nr. 4 has the impression of a 

 flower-bed planted with all kinds of brilliant growths. Around 

 a hill crowned by palms (see p. 76) there arise between green or 

 brown bushes of sea-weeds, here groups of white translucent 

 double -tubes, there single knobbed lumps, many looking like 

 ground-glass, and others as if made of wrinkled brown leather, 

 or again red sacks with two points, one above and one at the side. 

 From the walls hang down white tubes and many spots are over- 

 grown with crusts in which delicate star-shaped markings can be 

 detected. Here is still-life, as still as that of plants, so that the 

 life in it is hardly detectable. But these placid shapes too are 

 animals and an attentive look will show movements in the larger 

 examples: the opening or shutting of the two apertures. These 

 animals — called Ascidians, Tunicates or Sea-squirts — which 

 so much resemble plants, of all the invertebrate animals most 

 closely approach the vertebrates and have given rise to endless 

 controversies amiong zoologists studying the ancestry of the higher 

 animals. We will give now the most important points about their 

 structure and development. 



The Tunicates receive their name from the outermost layer 

 of the body, the so-called tunic or mantle. In Phallusia, the 

 largest species in the Gulf, often attaining 10 inches in length, 

 the tunic is thicked and knobbed and, as in all these animals, 

 consists of a substance which is chemically almost identical with 

 the woody substance (cellulose) of plants. In the mantle are 

 two openings; through the uppermost water streams in and comes 

 out again through the side one. The entering water goes into a 

 roomy cavity the walls of which consist of a network. Through 

 the meshes of the net the water passes into another chamber, 

 into which the excrements, eggs, etc. also issue, and thence passes 

 through the side opening to the exterior. The mesh-work just 

 mentioned is the gills. At the base of the large cavity commences 

 the gut, and the water serves both for respiration and to bring 

 nourishment: minute organisms and other food-particles being 

 led into the gut. The heart of the Tunicates has a remarkable 

 peculiarity. It does not drive the blood constantly in one direc- 

 tion through the body, as in other animals, but beats a certain 

 number of times first in one direction and then in the opposite, 

 so that the direction of the flood of blood is continually changing. 



The Tunicates are exclusively marine animals and almost all 

 sessile. They either remain single like Phallusia (fig. 29), the 

 translucent Ciona (fig. 30), which settles down of its own accord 

 in large numbers on the walls of the tanks, and the orange-red 

 Cynthia (fig. 28) — or they form colonies, in which the component 



