and within tliis narrow passageway as a food-charged 

 rope into the gullet and stomach. Thus the inverte- 

 brate chordates are fiher feeders, depending upon 

 plankton and detritus particles for nourishment. 



The Sea Squirts and 



1 heir K.111 (Suhphylum Uroclwrclata) 



Most sea squirts consist simply of a saclike body 

 permanently attached to some solid object or buried 

 shallowly in the ocean bottom. One body opening ad- 

 mits a current of water. The other serves for the 

 escape of the same current as well as of wastes and 

 reproductive products. 



The tadpole stage of most sea squirts could swim 

 through a buttonhole quite easily. Even in a shallow 

 dish with a black bottom, their tadpole-shaped bod- 

 ies are so transparent that it is easy to overlook them. 

 Yet the tail contains the complete notochord and the 

 slender nerve cord extending from a slightly enlarged 

 hollow brain in the dorsal portion of the body. A 

 light-sensitive simple eye and a minute organ of bal- 

 ance are embedded in the walls of the brain. 



The anterior end of the larval sea squirt is occu- 

 pied by the adhesive organs with which the creature 

 will attach itself at the time of transformation into 

 adult form. Already, however, it shows a small mouth 

 (incurrent opening) well forward on the dorsal sur- 

 face, leading into a capacious pharynx. Gill slits 

 through the pharyngeal walls communicate with the 

 atrium, which opens dorsally farther back on the 

 body. The small stomach is connected by a short in- 

 testine ending in the atrium, nearer the excurrent 

 opening from which water is discharged. 



When a sea squirt larva attaches itself and trans- 

 forms, it literally stands on its face while absorbing 

 and obliterating its tail, notochord, sense organs, 

 and so much of the nervous system that only a solid 

 ganglion remains, with nerves extending to the few 

 internal organs. At the same time the dorsal surface 

 becomes distorted through great enlargement of the 

 pharynx, until the incurrent and excurrent openings 

 are raised like two spouts on the squat body. Ex- 

 ternally the body surface secretes a covering of cellu- 

 lose as the tunic from which the attached animal 

 gains another common name, "tunicate" (Plate 

 144). 



When a beachcomber disturbs a sea squirt, the 

 creature usually contracts. On a rock between tide 

 marks this event is made obvious by two little jets of 

 water, one from the incurrent opening (mouth) and 

 the other from the excurrent (atrium). If a person 

 wearing slacks inadvertently steps beside a large sea 

 squirt on the beach, one or both jets may easily go up 



inside the trouser leg and reach the knee, to the 

 walker's sudden dismay. 



Along the Atlantic shores and also in California, 

 a common sea squirt with incurrent and excurrent 

 openings close together is Ciona intestinalis. Its pale 

 golden-yellow tunic and body wall are so transparent 

 that the inner organs can be seen through them. 

 The height of the slender body ranges from about 

 IVi to 2'/i inches. Its favorite sites for attachment 

 seem to be rocks, floats, and submerged timbers. 



Tethyiim pyri forme, the sea peach (Plate 143), is 

 of the right size and shape to earn its name, and 

 varies in color from orange to yellow, suflFused with 

 pink or red. It is a strikingly handsome member of 

 the coastal population from Maine northward in 

 cold, shallow water. 



Sea grapes are clusters of Mol^iila manhattensis, 

 the commonest sea squirt along North America's At- 

 lantic coasts from Massachusetts southward. Each 

 "grape" is almost spherical, about 1 inch in diame- 

 ter, and greenish yellow in color. The surface ap- 

 pears soft and spongy, and often serves as a site for 

 the attachment of other kinds of animals. 



Many sea squirts reproduce by budding as well as 

 by sexual means. They often build large, complex 

 colonies coating the surface of stones, sea walls, and 

 pilings (Plate 142). The various colonial species of 

 the genus Ainarouciiini are popularly called "sea 

 pork" from the translucent gray, tough tunic linking 

 one individual to the next. 



In addition to the attached sea squirts (class As- 

 cidiacea), the urochords include several types of 

 free-swimming pelagic animals. Appendicularians 

 (class Larvaceae ) never metamorphose from the 

 swimming, tadpole-like larval stage. Instead, they 

 develop reproductive organs and reproduce their 

 kind without ever "growing up." Their whole lives 

 are spent as minute creatures swimming in upper 

 levels of the sea, where they secrete complicated 

 food traps of mucus in the form of a lemon-shaped 

 house. Every few hours the trap is discarded because 

 it becomes clogged with particles unsuitable as food, 

 and the appendicularian spends about thirty min- 

 utes creating a new one into which it can move. 



Members of class Thaliacea are transparent ani- 

 mals that reach far larger size or group themselves 

 together into colonies big enough to handle easily. 

 Among the most spectacular of them are the pyro- 

 somes (Pyrosoiiia), found swimming gently in the 

 sea, either near its surface or far down in the depths. 

 Pyrosonui means "fire body," and refers to the fact 

 that in the dark these colonies can be detected as 

 luminous cylinders moving slowly through the water. 



Each translucent cylinder consists of hundreds or 

 thousands of '/s-inch sea squirts arranged radially 

 around a lengthwise central cavity, like the separate 

 parts of a pineapple around the hole where the core 



[287 



