THE TUBE-WITHIN-A-TUBE BODY PLAN 



191 



tained by catching a worm that has its 

 posterior end partly within its burrow. A 

 slow steady pull removes it, whereas a sud- 

 den one leaves the intruder with only a por- 

 tion of the worm in his hand, the other end 

 securely held in the burrow by the stiff 

 setae. 



There are no conspicuous sense organs on 

 the head end, like the eyes or tentacles of 

 Neanthes. Indeed, the animal seems to lack 

 a head, though it does have a protruding 

 "lip," the prostomium, which covers the 

 mouth. 



The saddle-shaped clitelhim rests on the 

 dorsal side about one-third of the way from 

 the anterior end. There are also several 

 openings which can be seen by careful in- 

 spection. Most noticeable are those of the 

 sperm ducts, which open on the fifteenth 

 segment. The fourteenth segment bears the 

 smaller openings of the oviducts, and each 

 segment except the first three and the last 

 bears a pair of nephridiopores. Finally, the 

 four openings which lead into the seminal 

 receptacles are located in the grooves be- 

 tween segments 9 and 10, and 10 and 11. 



Internal structures. A section of the body 

 wall (Fig. 10-16) shows the outer thin, 

 tough cuticle, which serves as a protective 

 layer for the tall columnar epithelial cells 

 composing the bulk of the epidermis. 

 Among these latter are scattered sensory 

 cells, sensitive to light, touch, and chemical 

 stimulation. Other cells dispersed among 

 the epithelial cells are the mucus-secreting 

 cells responsible for the slimy condition of 

 the skin, which is essential both for respira- 

 tion and locomotion. Beneath the epidermis 

 lie two layers of muscle, the outer circular 

 and the inner longitudinal. These function 

 in locomotion. Lying beneath the muscle 

 layers and lining the coelom is the peri- 

 toneum. The digestive tract is as it is in 

 Neanthes, making the tube-within-a-tube 

 plan conspicuous. By removing the dorsal 

 wall throughout the anterior half, the inter- 

 nal anatomy can be studied. The segmen- 

 tation which is so striking externally is just 



as conspicuous from the inside. Membra- 

 nous partitions, septa, which wall off each 

 segment, are perforated by the gut, nerve 

 cord, blood vessels, and nephridia. 



Beginning at the anterior end, the diges- 

 tive tract starts with the mouth, which 

 opens almost immediately into the large 

 muscular pharynx; this latter organ is used 

 as a kind of pump to draw food into the 

 mouth. Following the pharynx is the esoph- 

 agus, which opens into the crop, a storage 

 sac. This in turn leads into the gizzard, 

 which functions in the grinding of food, 

 much the same as a similar ortran in the 

 chicken. The remainder of the gut is a long 

 tube, the intestine. This organ bears a fold 

 along its dorsal side, the typhlosole, which 

 increases the surface of the 2ut without 

 increasing the volume of the animal. A 

 straight tube, from mouth to anus, suffices 

 for small animals, but larger animals re- 

 quire a tube with still greater surface area, 

 both for digestion and absorption of food. 



Various gland cells are located through- 

 out the digestive epithelium. Some produce 

 digestive enzymes, while the secretion of 

 others lubricates as well and thus facilitates 

 the movement of food. Lateral to the esoph- 

 agus and attached to it are three pairs of 

 calciferous glands, which function in se- 

 creting calcium carbonate for neutralizing 

 any acid soil that may be taken in with the 

 food. The epithelial lining of the gut se- 

 cretes fat-splitting, carbohydrate-splitting, 

 and protein-splitting enzymes. The gut is 

 surrounded by chlorogogen cells which are 

 derived from the peritoneum and probably 

 function in the elimination of wastes from 

 the blood. It is believed that fat is also 

 stored in these cells. 



Food for the earthworm consists of leaves 

 and any other available organic matter, 

 even bits of meat. Much soil is taken in with 

 the food and used later in the gizzard for 

 grinding the food in preparation for diges- 

 tion. Food is temporarily stored in the crop 

 before it passes into the gizzard, where it 

 is ground to a fine mass. It then passes on 



