THE TUNICATA OF THE GULF OF MEXICO 



By WiLLARD G. Van Name, American Museum of Natural History, New York City 



The Tunicata or Urochorda are a widely dis- 

 tributed group of exclusively marine invertebrate 

 animals mostly of small but not microscopic size, 

 many being less than an inch and few exceeding 

 3 or 4 inches in their maximum dimensions. 

 There are probably less than 900 valid species. 

 Evidently they are the survivors of what was a 

 much larger and more important group in past 

 geological times, although, as they are soft-bodied 

 creatures without a shell or hard skeletal parts, 

 they have not left recognizable fossils. 



Though of very minor economic importance 

 they are of great scientific interest, as of all the 

 invertebrates they show the closest relation to 

 the vertebrates and are included with them in the 

 phylum Chordata to which the higher animals, 

 including man, belong. In modern classifications 

 they are given the rank of a subphylum of the 

 Chordata. 



They are, however, a group that has undergone 

 retrograde instead of progressive evolution. 

 Though the adult tunicates are creatures with a 

 rather low and simple type of organization so that 

 the older zoologists considered them to be shell- 

 less moUusks, yet in their larval stage they have 

 so much resemblance (except for much smaller 

 size) to the tadpoles of the amphibians (frogs and 

 salamanders) in certain important structural 

 characters that we cannot dismiss it as mere 

 coincidence and must regard it as indicating 

 common ancestry at some time in the remote 

 past. 



Three classes are included in the Tunicata, two 

 of which, the Thaliacea and Larvacea, are free- 

 swimming pelagic forms and are few in species. 

 The third, the Ascidiacea (ascidians), is much the 

 largest and best known class and will be consid- 

 ered first. 



The ascidians are, except a few that embed 

 themselves in the sand or mud of the sea bottom, 

 permanently attached animals when adult and 

 depend for food on the minute organisms that 



the tides and currents bring to them. Their sac- 

 like body of oval or more or less irregular shape 

 is enclosed in a thick outer tunic called the test, 

 which is sometimes gelatinous but more often of 

 tough, leathery consistency. 



In it there are only two openings to the outside, 

 both of which are often extended into short tubes. 

 One of the tubes serves as the mouth for the en- 

 trance of water for respiration and also brings in 

 their food. The other is the excurrcnt aperture 

 for the discharge of the water, the waste matter, 

 and usually the eggs or larvae. All ascidians 

 reproduce sexually by means of eggs and sperma- 

 tozoa, but very many of them, especially those in 

 which the individuals are of quite small size, also 

 reproduce asexually by budding. The new indi- 

 viduals thus formed remain attached to and en- 

 closed with the parent in a common mass of test 

 so that what is termed a colony of many united 

 individuals is formed. Such species are called 

 compound ascidians; the small members that com- 

 pose the colony, the zooids. Species that do not 

 bud are distinguished as simple ascidians. Com- 

 pound ascidians may be recognized by the many 

 small apertures belonging to the numerous zooids 

 and often have much superficial likeness to the 

 sponges in association with which they very often 

 grow. 



ASCIDIAN FAUNA OF THE GULF OF 

 MEXICO 



As far as the ascidians are concerned the Gulf 

 of Mexico is not a distinct faunal region but a part 

 of that which is commonly called the West Indian 

 region, though actually much larger, including all 

 the eastern American tropical and subtropical 

 waters from the Carolinas on the north, with an 

 outpost at Bermuda, to far south on the coast of 

 Brazil. Throughout this vast region tliere is 

 considerable uniformity as far as many of tlie 

 commoner species are concerned, and even rare 

 and locally distributed species may appear in 



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