The Colour Sense. 35 



inherited memory, though not, as we believe, in the fulness with 

 which it must ultimately be acquired. 



This, however, seems certain, that the development, not only of 

 the sense organs, but of organs in general — that is, the setting aside 

 of certain portions for the performance of special duties, and the 

 modifications of those parts in relation to their special duties, is 

 closely related to the activity of the organism. Thus, we find in 

 those animals, like some of the Coelenterata, which pass some 

 portion of their existence as free-swimming beings, and the remainder 

 in a stationary or sessile condition, that the former state is the most 

 highly organized. This is shown to a very remarkable degree in the 

 Sea Squirts (Ascidians), a class of animals that are generally 

 grouped with the lower Mollusca, but which Prof. Ray Lankester 

 puts at the base of the Vertebrata. 



These animals are either solitary or social, fixed or free ; but 

 even when free, have little or no power of locomotion, simply float- 

 ing in the sea. Their embryos are, however, free-swimming, and 

 some of the most interesting beings in nature. Some are marvell- 

 ously like young tadpoles, and possess some of the distinctive 

 peculiarities of the Vertebrata. Thus, the body is divided into a 

 head and body, or tail, as in tadpoles. The head contains a large 

 nerve centre, corresponding with the brain, which is produced back- 

 wards into a chord, corresponding to the spinal chord. In the 

 head, sense organs are clearly distinguishable ; there is a well- 

 marked eye, an equally clear ear, and a less clearly marked olfac- 

 tory organ. Besides this, the spinal-cord is supported below by 

 a rod - like structure, called the notochord. In the vertebrate 

 embryo this structure always precedes the development of the 

 true verteb"al column, and in the lowest forms is persistent through 

 life. 



We have thus, in the ascidian larva, a form which, if permanent, 

 would most certainly entitle it to a place in the vertebrate sub- 

 kingdom. It is now an active free-swimming creature, but as 

 maturity approaches it becomes fixed, or floating, and all this pre- 

 figurement of a high destiny is annulled. The tail, with its nervous 

 cord and notochord atrophies, and in the fixed forms, not only do 

 the sense organs pass away, but the entire nervous system is 

 reduced to a single ganglion, and the creature becomes little more 

 than an animated stomach. It is, as Ray Lankester has pointed out, 

 a case of degeneration. In the floating forms, which still possess a 



G 



