defined, large open spaces in the body. They not only lack a 

 body cavity, as is the case in all Platyhelminthes, but they also 

 do not possess a digestive cavity with a lining of epithelial cells. 

 The act of digestion is carried out in the central part of the body 

 in a tissue which appears under the microscope as a meshwork 

 of vacuolated cytoplasm with scattered nuclei. This central 

 (digestive) parenchyma is surrounded by a denser tissue, the 

 peripheral parenchyma, in which are imbedded the organs 

 making up the hermaphroditic reproductive system of these 

 animals. Surrounding the peripheral parenchyma, and more or 

 less penetrating it, are several layers of muscle fibers and various 

 epithelial gland cells. The exterior of the body is clothed by a 

 layer of ciliated epithelial cells. The muscle fibers, epithelial 

 gland cells, and outer epithelium form a complex that gives stiff- 

 ness and preserves the shape of the animal. The German term 

 "hautmuskelschlauch" (outer muscle sheath), often used in the 

 literature, applies only to the layers of muscles, but the epithe- 

 lium and its often in sunken nuclei and derived gland cells 

 together with the muscle layers make a more or less interwoven 

 unit which can be thought of as an outer body wall. The brain 

 (cerebral ganglion) and large nerve chords lie in the peripheral 

 parenchyma, or, in a few species, in the base of the outer 

 epithelial layer. There are no skeletal or excretory systems. As in 

 many marine fiatworms, protonephridia are not present. 



The acoels are small animals from ~0.5 to ~ 10.5 mm in length, 

 mostly oval to elongate oval in shape and without striking sur- 

 face features such as appendages or special markings. They tend 

 to be more or less transparent or white or pale in color, although 

 a few species are conspicuously colored by the presence of sym- 

 bionts or special pigments. The lack of a gut cavity as well as of 

 defined body spaces, usually the lack of conspicuous internal 

 organs aside from those of the reproductive system, and the 

 presence of a uniform parenchyma around the organs give them 

 a homogenous appearance that enables the animals to be rather 



easily distinguished from other Turbellaria. Under low 

 magnification, other characters are visible which aid in 

 distinguishing acoels, namely the presence of a statocyst, the 

 simple mouth opening without a pharynx or with a very simple 

 tubelike one, and the arrangement of large eggs full of stored 

 food material (Fig. 1). Location of the mouth and location and 

 arrangement of sperm and the male copulatory complex (and of 

 stored sperm in the female parts of some species) are useful for 

 recognizing families and genera. Diagnostic characters which 

 require observation with the higher powers of the microscope or 

 recourse to the making of histological sections include details of 

 the reproductive system, presence or absence of frontal or other 

 glands, arrangement of muscle fibers in the outer body wall, and 

 type of nervous system. 



The presence of a statocyst is not unique to the acoels but 

 this, together with the lack of a conspicuous pharynx and 

 digestive tract and the presence of large egg cells, is a certain clue 

 to their recognition. The circular clear vesicle of the statocyst 

 with an enclosed statolith is easy to see in the anterior end of the 

 living animal, but it is harder to find in fixed material. In sec- 

 tions it is found lying close to or imbedded in the anterior 

 (cerebral) ganglion. 



The mouth in acoels is difficult to see, since in most species it 

 is simply a break in the outer epithelial layer closed by a few 

 sphincter muscle fibers. It opens directly into the central 

 digestive parenchyma (Fig. 2). In relatively few genera, there is a 

 simple tube-shaped pharynx (pharynx simplex) (see Figs. 7, 8, 9, 

 23a, 24a)2 which is essentially an invagination of the outer body 

 wall layers. This lack of a speciaUzed pharynx is characteristic of 

 the Acoela as compared with the other orders of Turbellaria, 



^References in the remainder of this section, unless otherwise indicated, are to the examples 

 in the key where the structures are listed or are indicated by arrows on the figures. 



frontal glancj 



male germina 

 center 



mouth 



mature egg 



sperm entering 

 seminal vesicle. 



statocyst 



.female 



germinal center 



seminal bursa 



copulatory organ 



Figure 1.— Generalized diagram of moel siruclurc. as seen in surface vie» , when llie lurhellarian is sHimmin|> ur ijuiel under a co>erslip. 



