is considered a true seminal vesicle (Figs. 18, 26a, 91); inside of 

 this type there is usually present, besides the sperm, a granular 

 secretion supplied by the walls of the vesicle or by associated 

 gland cells. 



Arrangements for the ejaculation of sperm in the acoels range 

 from a simple outside opening at the end of a true or false 

 seminal vesicle to more complex situations that include a 

 passage to the outside (male antrum) and a well-developed 

 penis. The penis, when present, may be composed of various 

 arrangements of fine or coarse needles (Figs. 32b, 54, 106), may 

 consist of a cone-shaped group of muscles (Fig. 107), or may be 

 a glandular or muscular tube (Figs. 91, 94, 115). In some cases, 

 it is inverted into the seminal vesicle itself when at rest and 

 everted to the exterior when sperm are to be discharged. It may 

 be enclosed by a penis sheath or sac which may also enclose the 

 seminal vesicle. A special feature of some families is cuticular 

 needles (adenodactyls) which do not make up a penis, but which 

 are associated with the reproductive passages and which 

 apparently have a stimulatory function (Figs. 37, 38). 



The sperm of acoels are biflagellate (Hendelberg 1977) and 

 internal fertilization is the rule, as in other Turbellaria. The 

 arrangement of the reproductive organs suggests cross fertiliza- 

 tion. Behavior that suggests mutual insemination has been 

 observed a number of times and it has been shown conclusively 

 to be the case by histological sections of animals in coitus 

 (Hyman 1937, in Amphiscolops langerhansi; Westblad 1946, in 

 Conaperta jlavibacitlum). 



Asexual reproduction has been reported several times, but has 

 been shown clearly to be a normal method only for the species 

 Paratomella unichaeta (Dorjes 1966) and Pseudohaplogonaria 

 macnaei (du Bois-Reymond Marcus 1957). Acoels have rather 

 limited powers of regeneration, a characteristic which is un- 

 doubtedly associated with the lack of asexual reproduction. A 

 summary of studies on regeneration in this order is given by 

 Steinbock (1967) and should be referred to for its bibliography 

 as well as for his view of the evolutionary significance of exper- 

 iments in this area. 



Gland cells and their secretions play an important role in the 

 physiology of acoels judging by their large numbers and 

 frequent occurrences. Some types of gland cells are so con- 

 spicuous as to be useful in the identification of species. Most ob- 

 vious are those whose secretions are released onto the surface of 

 the body. Rhabdites are common, and under the microscope 

 they can be seen as small elongate bodies lying in or close to the 

 outer epithelial layer. They are produced in the epithelium or in 

 special epithelial cells which are sunken below the surface but 

 which retain their connections to the surface so that the rhab- 

 dites may be extruded to the outside. Rhabdites may be 

 scattered irregularly over the body, be present only in limited 

 areas, or be arranged in beautiful rows running the length of the 

 animal. Often they are colorless, but they may carry color that 

 gives a characteristic hue to the entire animal. Rhammites are 

 more elongated or irregular bodies lying deeper in the interior of 

 the body. 



A group of gland cells makes up the frontal gland which is 

 characteristic of and occurs in most acoels as well as in some 

 other turbellarians. This gland may not be conspicuous in living 

 animals, but in stained preparations it appears as a varying 

 number of differentially staining cells with ducts opening close 

 together at the anterior tip of the body. In some cases, the ducts 

 unite to form a small ampule before opening to the outside. 

 Studies of the detailed structure of this gland suggest that it is in- 



volved in chemoreception as well as in secretion (Antonius 

 1970). 



Other gland cells are present in association with the reproduc- 

 tive organs. There do not appear to be any special digestive 

 glands, although some gland cells may be developed around the 

 mouth or pharynx (Fig. 14). Adhesive glands in conspicuous 

 papillae, as in some other turbellarians, are not usually present, 

 but groups of special adhesive cilia, "haptocilial' in Hesioliciurn 

 inops, have been described recently (Tyler 1973; Crezee and 

 Tyler 1976). Some cells in the parenchyma that produce pigment 

 would be classed as gland cells. 



In most species a layer of circular muscle fibers lies to the out- 

 side next to the epithelium with alayer of longitudinal fibers just 

 inside this (Fig. 130). In the few genera where this arrangement 

 is reversed, it is considered to be of taxonomic significance and 

 has been used to define at least one genus (Fig. 128). There may 

 also be a third layer of fibers arranged diagonally between, or 

 close to the others, and sometimes there is an additional layer of 

 circular fibers (Bush 1975). Other fibers, probably derived from 

 these, form the sphincter around the mouth, and, in the 

 reproductive system, muscle fibers are arranged as sphincters, 

 retractors, protractors, and as sheets in the walls of some 

 organs. In many acoels, muscle fibers that extend through the 

 parenchyma from side to side are only weakly developed so that 

 movement is restricted mostly to elongations or shortenings of 

 the body with only a limited amount of twisting or coiling. 

 Locomotion in these species is a characteristic gliding movement 

 accomplished by means of the cilia on the surface. However, in 

 some species, intraparenchymal muscles are better developed 

 and act as retractors of the anterior or posterior ends, may be 

 used in rolling the sides of the body ventrally as when capturing 

 prey, or, in some interstitial species, provide more varied coiling 

 and twisting of the body as the animals move about in spaces 

 between the sand grains. 



The nervous system of acoels has been described in relatively 

 few species since the nerve tissue is usually not clearly differen- 

 tiated in routine sections. In most acoels, however, it can be seen 

 that there is a more or less complex cerebral ganglion lying in 

 the parenchyma at the anterior end of the body with three to six 

 nerve chords extending posteriorly. Some smaller nerves extend 

 to the surface at the anterior end, to the statocyst, and to the 

 neighborhood of the frontal gland. In a few species of acoels 

 and in the order Nemertodermatida, all or part of the nerve 

 tissue, instead of lying in the parenchyma inside the outer wall 

 muscle layers, is found instead lying next to or even somewhat 

 intermeshed with the bases of the outer epithelial cells. Such an 

 epithelial nervous system is considered by some authors to repre- 

 sent a primitive situation and is, therefore, held to be of impor- 

 tance in plotting the evolutionary changes and relationships of 

 the Turbellaria. For differing opinions on this point, see discus- 

 sions by Ax (1961) and Karling (1974). 



Sensory structures appear to be rather few and simple, but, 

 since most of them must be represented by single cells, they are 

 not usually identified and are probably more numerous than is 

 realized. Observations of the reactions of living animals indicate 

 that among the cilia there may be sensory hairs, but acoels do 

 not have the large conspicuous sensory hairs, or the ciliated pits 

 or grooves found in some other turbellarians. Only a few species 

 have pigmented eye spots or pigment-cup ocelli. Electron 

 microscopy will undoubtedly give needed information on sen- 

 sory structures (e.g., the recent work by Crezee and Tyler (1976) 

 on the caudal organ in Hesioliciurn inops). 



