128 INTRODUCTION TO CLASSIFICATION. 



there are " cystic " asexual conditions of most, if not all, the 

 species. The Tasniada are internal parasites. 



3. The TDRBELLARIA are non-parasitic. Their flattened or 

 elongated bodies are covered with cilia throughout life, and are 

 not provided with suckers. They may be hermaphrodite or uni- 

 sexual. To this order belong the genera Planaria and N emeries. 



4. The ACANTHOCEPHALA are unsegmented, vermiform, in- 

 ternal parasites (Echinorhynchus), devoid of any alimentary canal, 

 and adhering by means of a proboscis, which is covered with 

 spines. The sexes are distinct, and there is only one set of repro- 

 ductive organs in each individual. 



5. The NEMATOIDEA (or Thread Worms, including the 

 Gordiacea) have vermiform non-ciliated bodies, devoid of seg- 

 mentation. The integument is dense, and beneath it lies a 

 single layer of muscles, usually divided into four longitudinal 

 bands. The intestine is straight, extending from the mouth, 

 at the anterior end of the body, to the anus, which is more 

 or less near the posterior end. In some few species the intes- 

 tine is abortive. The sexual organs are tubular glands, open- 

 ing externally by a single aperture, and are usually lodged 

 in distinct individuals. The embryo at once takes on the 

 form of a worm. 



There are both free and parasitic Nematoids, and some 

 species pass part of their lives in the one condition, and part in 

 the other. 



6. The EOTIFERA (or Wheel-animalcules) have subcylin- 

 drical bodies, more or less distinctly annulated. The oral end 

 is provided with cilia, arranged to form the chief locomotive 

 organs, but there are no cilia on the rest of the body. The 

 alimentary canal is provided with a peculiar gizzard. The 

 sexes are distinct, the males being frequently smaller than the 

 females, and having the alimentary canal abortive. 



The ECHINODEEMATA may be divided into seven orders 



1. The ECHiNiDEA(or Sea-urchins). The body is spheroidal 



or discoidal. There are five main ambulacral tubes, which 



extend from the mouth to the apex, or summit of the opposite 



side of the body. The corona or main part of the skeleton is 



