FLATWORMS 



differentiated sense organs are present, such as the eyes of some species, and 

 special regions of sensory epitheUum. 



The reproductive system consists of paired, sac-like ovaries or testes lying 

 between the lateral pouches of the digestive tract. Each gonad opens ex- 

 ternally by a separate pore. Some nemertines are monoecious, but in most 

 species the sexes are separate. According to the species, fertilization may 

 occur within the body of the female or in the open water after the discharge 

 of ova and spermatozoa. Development usually includes a ciliated, free- 

 swimming larva, the pilidium (Fig. 11.23), which later metamorphoses to form 

 the adult. In a few species the young develop into miniature adults within 

 the body of the parent. 



The Nemertinea are thus bilateral, triploblastic animals without a body 

 cavity and with protonephridial excretory organs. To this extent they 

 resemble the Platyhelminthes. On the other hand, they have an anal open- 

 ing, a blood-vascular system, a more highly developed nervous system, and 

 a unique feature, the proboscis. Considering also the fact that the course of 

 embryonic development differs in the two groups, their separation into two 

 phyla seems completely justified. These phyla can be placed together in the 

 series Acoelomata as animals which are triploblastic and bilaterally sym- 

 metrical but lack a body cavity (see Fig. 7.4, p. 221). 



The Flatworm Body Plan 



As compared with any member of the phylum Platyhelminthes, the 

 Nemertinea are obviously more highly specialized in a number of ways. The 

 nemertines may be thought of as the culmination of evolution within the 

 acoelomate line; as specialized end forms, they are not of particular significance 

 in considering the typical flatworm plan. Most of the Platyhelminthes are 

 also highly specialized types, having been modified extensively through long 

 periods of parasitic existence. For comparative purposes, as stated earlier, 

 the free-living Turbellaria are most significant. Their plan of organization 

 shows an animal which is triploblastic, with epidermis, gastrodermis, and 

 intervening mesenchyme, in contrast to the diploblastic organization of the 

 simplest coelenterates. The dorsoventral differentiation, the anteroposterior 

 differentiation with its correlated bilateral symmetry and development of a 

 head, the sensory-neuro-muscular system, and the reproductive system with its 

 complex parts are all evidences of greater specialization than anything found 

 in coelenterates. The turbellarian can be compared with a coelenterate such 

 as a medusa, or a shortened polyp creeping upon its oral surface, but the 

 more obvious comparisons are with higher animals. The great diversity of 

 forms characteristic of the series Acoelomata presents evidence of the plasticity 

 or adaptability of this type of construction. The fact that all higher animals 

 possess some kind of a body cavity, however, indicates that the solid-bodied 

 construction of acoelomates has imposed limits upon evolutionary progress. 



343 



