130 UNDERWATER GUIDE TO MARINE LIFE 



Sea Lilies: Class Crinida — Figure 42 



These are almost exclusively deep-water animals and are among the most 

 common of animals found there. Deep-water sea lilies are stalked and very 

 plantlike. There are a few sea lilies that break off their stalks and swim by 

 waving their arms. One of these is the sea feather or feather star, Antedon, and 

 it sometimes ranges to waters only about 100 feet deep. It is of temperate 

 distribution. 



Sea Cucumbers: Class Holothurida — Figure 42 



The onlv echinoderms that have lost their spinv or hard skin are the sea 

 cucumbers, which have an appearance exactly like their name. They exist in 

 shallow and deep waters all over the world and may be either chunky and 

 muscular or elongate and delicate. A ring of tentacles encircles the mouth. 

 The tentacles are tactile and chemoreceptive organs. Tentacles also serve to 

 shovel mud and sand, the sea cucumber's source of food, into the mouth. 

 Some sea cucumbers have the defense mechanism of eviscerating themselves 

 through the mouth by means of a violent contraction of the body when they 

 are disturbed. The sea cucumber soon regenerates the internal organs. The 

 beche-de-mer, Holothnria (/ig. 43), is a common form found in tropical seas 

 the world over. (Its commensal relationship with the pearlfish is described in 

 Chapter 10 under that fish.) 



INVERTEBRATE CHORDATES: Phylum Chordata 



This is the phylum to which the vertebrates belong, but there are some 

 invertebrate members of this phylum. These invertebrates are the species that 

 have no backbone, hence are truly invertebrates. But they do have other internal, 

 anatomical characters that vertebrates have (including a dorsal, tubular nervous 

 system), and these place the invertebrate chordates or prochordates in the same 

 phylum as vertebrates. All of the prochordates are filter-feeders, and most of 

 them are small. 



Cephalochordates : Class Cephalochordata — Figure 44 



These are the lowest forms of fishlike animals. Avifhioxus and its close allies 

 are small, up to 3 inches or so, and are common buried in clean, sandy shores 

 of much of the world. They lie in the sand with the mouth protruding, filtering 

 plankton from sea water by means of a large basketlike pharynx. 



Hemichordates : Class Hemichordata — Figure 44 



The acorn worms, Balanoglossus, and others, live in holes in soft-bottomed 

 shores and feed, as do other prochordates, by filtering sea water. The acornlike 

 "head," which is used both in burrowing and feeding, separates them from other 

 wormlike animals. Some members of this class are small, sessile, and colonial. 

 They live on the sea bottom and are not likely to be encountered by the diver. 



