42 Laboratory Guide in Zoology 



face notice the space formerly occupied by the peri- 

 proct. Notice that around it are five genital plates, 

 each having a small hole near the apex ; these are the 

 reproductive openings. See that one of the plates is 

 larger than the others and is full of minute perforations ; 

 this is the madreporic body. Make a drawing of the 

 aboral surface of this specimen. 

 XL — Study the oral surface of the same specimen. Notice 

 the large opening formerly occupied by the peristome. 

 What shape is it ? Notice the five interradial sutures' 

 from the tip of each genital plate to the corresponding 

 angle of the mouth. pentagon. See the double rows of 

 polished prominences for the attachment of the spines. 

 See also the zigzag ambulacral suture running between 

 each double row, and the double rows of ambulacral 

 pores on each side of the double rows of spines. 



THE SEA-CUCUMBER 



The Holothurians or Sea-cucumbers are strange crea- 

 tures that live in the ocean. They form a separate class 

 (Holothuroidea) of the great group to which the starfish 

 and sea-urchin belong. They have a water-vascular sys- 

 tem similar in general to that of the starfish. 



I. — Make a drawing showing the general shape of the body 



and the position of the tentacles, as well as of the mouth 

 and of the anal opening. 



II. — Describe the number, size, shape, and position of the 

 tentacles. 



III. — Examine with a hand lens the ambulacral suckers with 

 which the body is covered. Make a drawing of one of 

 these, considerably magnified. Are these more abun- 

 dant on the lower side of the animal ? On the upper 

 side, are they more abundant near each end than in the 



