202 



ASCIDIANS OF GUERNSEY. 



ganglion found in the adult. The sensory organs disappear, 

 but the alimentary canal and gonads increase greatly in size. 

 Thus the adult form gives us no hint whatever as to its rela- 

 tionship with the vertebrates. 



Before their life history was known these Ascidians were 

 classed with the Polyzoa (Sea-mats) and Brachiopods (Lamp- 

 shells) under the name of Mulluscoida. They were considered 

 to be relations of the Molluscs, but later research has placed 

 them much higher up the scale of animal life. This is a good 

 instance of the importance of knowing the complete life- 

 history of an animal before referring it to one or other of the 

 great groups of animals. 



No mention has been made of that remarkable creature 

 known to naturalists as the Lancelet ( Amphioxus lanceolatus ). 

 This has affinities with both the Ascidians and the higher 

 vertebrates. It is a fish-like, free-swimming animal about 

 three inches long, of transparent structure. It spends most of 

 its time in the sand but cat) swim on occasion. It has a noto- 

 chord stretching from head to tail, besides a dorsal nerve 

 tube, which is remarkable for the fact that it contains sensory 

 spots inside it. The notochord stretches the entire leugth of 

 the body, instead of stopping off about the middle of the cra- 

 nium as in the higher Vertebrates. Although these structures 

 are permanent, and not only larval as in the Tunicates, there 

 is a great gap between it and the lowest of the fishes. It has 

 no distinct cranium and so naturalists have proposed to esta- 

 blish for it a class called the Acraniata, while the fishes and 

 other higher forms compose the Craniata. 



Turning now from the book to the seashore we must look 

 in many different places to find all the Tunicates in their 

 natural homes. They are ubiquitous, having been found in 

 all seas from the Arctic regions to the Tropics, and from 

 between tide-marks to a depth of over 2,000 fathoms. As an 

 indication of the deep water in which these soft-bodied crea- 

 tures can live, it may be stated that HypobytJiius calycodes 

 was obtained in the N. Pacific at 2,900 fathoms or over three 

 miles vertical depth, while Abyssascidia Wyvillii came up 

 from 2,600 fathoms off the South of Australia. Both of these 

 species are simple forms. 



But although many species live in deep water, the rocks 

 fairly low down the tide range will be found to yield many 

 species of Ascidians, while seaweeds and stones are often 

 covered profusely with the compound forms. Vermilion 

 patches are frequently seen coating comparatively large areas ; 

 these are colonies of Lcptoclinum lacazii. Mr. Sinel of Jersey 



