Tunicata. 87 



after attaining a certain size. Like Pyrosoma the Salpae are phosphor- 

 escent animals. 



In connection with the Tunicata we may mention the Lancelets, 

 which are looked upon as the lowest vertebrate animals. The Neapolitan 

 Lancelet, Amphioxus lanceolatus (Fig. 66), is scarcely 2 inches long, 

 colourless and translucent ; its only skeleton is a notochord ; it has no 

 head. Instead of a heart it has pulsating vessels containing colourless 

 blood, and like the Tunicates it has a throat perforated with many 

 gill-slits. But its development is even of greater interest to zoologists 

 than its anatomy; for it resembles greatly that of the Ascidians cp. 

 p. 85 and points to a near relationship of these two forms. It is sup- 

 posed that there must once have existed a group of animals possessing a 

 notochord and gill-slits, whose descendants are now represented by three 

 clans. 1) the Fishes and Terrestrial Vertebrates, (2) the Lancelets, (3) 

 the Tunicates. 



Amphioxus lives in the sand of flat coasts and resembles a worm 

 more than a fish. It is found in thousands along the beach of Posilipo 

 and similar tracts of the Bay of Naples. It was first discovered on 

 British coasts by Mr. Couch in 1831; within the last few years several 

 kinds of Lancelets have been found in other seas. 



In the Aquarium it can only be kept, if a plentiful supply of sand 

 be in the tank (the little open one in front of tank Nr. Hi . In this 

 it burrows immediately and only comes out at night or if disturbed. 



FISHES (PISCES). 



Fishes have such well-known external characters , that they will be 

 rarely mistaken for members of the other large groups of the animal 

 kingdom. We will only remind the reader that the "cuttlefish" are not 

 really "fishes" in the zoological sense but mollusks. and that there are 

 a few true fishes differing considerably from the usual form, such as the 

 snake-like eels, the flattened rays and soles, and the graceful sea-horses. 



As the fishes, like the other animals, are arranged in the Aquarium 

 so as to suit their various modes of life, we shall also arrange our 

 remarks to illustrate the same. Scientifically the fishes are divided into 

 cartilaginous and bony fishes respectively, but in both groups according 

 to their habits we can distinguish Drift-fishes, that is such forms as 

 are always or almost always swimming, and Bottom-fishes, which pass 

 the greater part of their life lying on or even buried in the sand, or on 

 the rocks. This distinction cannot, of course, be carried through quite 

 consistently, as we find all sorts of intermediate forms. 



We will begin with the Cartilaginous Fishes, to which the Sharks, 

 Dog-fish and Kays belong. Those in the Aquarium are chiefly Bottom- 

 fishes, which display only very little of their life. Let us turn our 

 attention first of all to the Sharks. 



The word "Shark" will cause the reader to think at once of those 

 gigantic robbers of the ocean. w r hich have become the terror of the sai- 



