CHAPTER IX 



The Acorn Worms 



and Their Kin 



( Pliylu m Hem ic/iordafa ) 



An acorn worm in its 

 burrow in the sea bottom 



A, 



.MONG the treasures to be found in sand and 

 sandy mud along the world's seashores are fragile, 

 pinkish tan animals called acorn worms. At first 

 glance each might be thought to be a pale, soft-bod- 

 ied earthworm. The 5- to 6-inch body even wears a 

 swelling near the anterior end, suggesting the clitel- 

 lum of an earthworm. But the body of an acorn 

 worm is not segmented, and the enlarged region is a 

 collar that extends completely around it. 



Sometimes an acorn worm is exposed when a 

 stone, half-sunken in the bottom, is lifted. One wall 

 of the worm's burrow is taken away. These creatures 

 build branching U-shaped or Y-shaped tunnels in 

 the bottom sediments, lining them with mucus. At 

 night an acorn worm may emerge from its burrow 

 and creep over the bottom among eelgrass or other 

 plant tangles, but by day it is almost sure to be out of 

 sight. 



In spite of the wormlike body, acorn worms pos- 

 sess a feature that, until recently, was regarded as 

 earning them a place in phylum Chordata, as de- 

 generate relatives of the vertebrates. Between the 

 pharynx region of the digestive tract and the outside 

 of the body, acorn worms and some of their close kin 

 show a series of paired openings. Clefts of this kind 

 are known otherwise only among the chordates 

 and, possibly, one extinct genus of echinoderms. 



Today the phylum name Hemichordata is re- 

 garded as suggesting a sort of halfway station, not 

 really eligible for inclusion among primitive chor- 

 dates but rather worthy of a phylum by itself. Similar- 

 ities between embryonic stages of hemichordates and 



echinoderms may indicate a closer link with sea stars 

 and their kin. 



The first part of an acorn worm, anterior to the 

 collar region, contains a contractile chamber serving 

 as a heart. It draws blood from a dorsal longitudinal 

 vessel, pumps it through an organ assumed to serve in 

 excretion, thence around the digestive tract on each 

 side, to join into a ventral longitudinal vessel. This 

 first part of the body is used also in burrow-making 

 and in pulling the body along when the animal is ex- 

 posed on the surface. Cilia, which cover the body, 

 help in a slower, gliding kind of locomotion. 



The mouth opens below the forward end of the 

 collar and leads into a long, straight digestive tract. 

 The anus is at the posterior end of the body, and 

 there is no postanal tail as in chordates. Just behind 

 the collar, the gill openings from the pharynx dis- 

 charge water taken in through the mouth. This copi- 

 ous flow is directed from the dorsal surface into the 

 worm's tunnel or the surrounding sea. 



In some species of the genus Balanoglossus. the 

 first part of the body and the collar together might 

 suggest an acorn in its cup; from this the most familiar 

 of the hemichordates receives a common name. In 

 Saccoglossits the first part of the body is greatly ex- 

 tended. The twenty-odd species of Balanoglossus in- 

 clude B. clavigerus from the Mediterranean and B. 

 aiirantianis from the coast of the Carolinas. Sacro- 

 glossiis kowalevskii is found on both sides of the 

 North Atlantic and is probably the acorn worm most 

 frequently encountered. 



Members of the genus Ptychodera resemble Ba- 



146] 



