484 NATrRAL TTISTORY OF HAWAII. 



a few shells of apparently the same species, gathered on Laysan Island. They 

 are of more than ordinary interest, since they represent a gronp which was very 

 abundant in early geologic times and of whicli a few t>'pe-forms have persisted. 

 almost unchanged, to the present day. 



Sea-Squirts. 



The sea-squirts or aseidians form a class °^ that is well represented on the 

 reef and the sea-bottom offshore. To look at the shapeless sack-like mass of 

 a leathery or gelatinous texture having two openings through which, in the 

 living creature, currents of water enter and leave the body, no one would for 

 a moment believe that they could claim even a remote relationship with the 

 typical vertebrate animals. Nevertheless, they are placed by modern zoolo- 

 gists in a phylum "'■' with several other unusual animals thought to form a 

 group ■"'■'' more closely allied to the typical vertebrates than to any of the many 

 varied tyjjcs of invei'tebrate animals. In order to trace the affinities, how- 

 ever, the larval condition of the sea-squii-t must be studied, as they are 

 strangely degenerated animals in the adult form. They begin life as a free- 

 swimming tadpole-like lai'va:- which approaches somewhat to the vertebrate 

 type of structure. That is, they possess a notochord,^^ a central nervous 

 system, gill slits, and certain other fundamental characteristics. As they 

 attain the adult condition, however, radical changes in form and structure take 

 place. They usually find a suitable location and become attached, remaining 

 for their life-time in one place, firmly adhering to various objects, as a shell. 

 a coral, a rock, or a bit of seaweed. The pelagic free-swimming sea-squirts 

 or salpa I have never seen in Hawaii, but there are both simple and compound 

 aseidians in abundance about the islands. Among the compound fixed types, 

 the colonies, as they are called, are produced by budding from a single parent 

 animal. They commonly form jelly-like incrustations in which a whitish star- 

 like pattern can be seen. They abound on the under side of submerged objects, 

 or on seaweeds. But the simple forms, being much larger, are sure to attract 

 the notice of the naturalist on the reef. On being lifted from the water, at- 

 tached to a stone or coral, they squirt a small stream of water from the open- 

 ings as they contract. 



The B.vlanoglossus. 



A curious soft-bodied worm-like animal,''' whose claim to a place among 

 the CJionJaia rests upon the fact that an outgrowth of the intestine extends 

 into the probosis, where it foi-ms a solid rod which, in its origin, suggests the 

 notochord in more typical forms, occurs in the sand in shallow water along our 

 shores; in pockets in the reefs, as, for example, in the reef at Kahala, Oahu. 

 Specimens may be secured by passing the sand through a sieve, or dredging it 



oaTujiicatn. b* Chordata. ^'' Protnvertebratn. 



■^■^ A doi'sal longitudinal rod of supporting cells thnt corresjjonds to the primitive backbone. 



^^ Balanoglossvs. 



