THE INFUSORIA 441 



this dimorphism has no relation to sex, but only to differences in the sur- 

 rounding medium ; he states that by the addition of quinine. (1 : 80,000) to a 

 culture of the small forms he was able to bring about the appearance of the 

 large forms. 



The body of a ciliate is often prolonged into processes, spikes, etc., giving 

 the animal a curious appearance. The most bizarre forms are found amongst 

 the species entozoic in the digestive tracts of ruminants, such as Ophryoscoler, 

 Entodinium, etc. ; but some free-living species also exhibit peculiarities of 

 external form. Actindbolus radians (Hplotricha) has the body covered with 

 tentacle-like processes, each bearing a trichocyst at the extremity. Legendrea 

 loyezce (Faure-Fremiet), allied to Prorodon, bears on the left side of the body 

 about twenty digitif orm .processes of variable length, flexible but not motile ; 

 each process is composed of clear protoplasm enveloped by the pellicle, and 

 at its slightly dilated extremity is lodged a bundle of trichocysts. HasiateUa 

 radians is a free-swimming Vorticellid which bears two circlets of pointed 

 " fulcra," or spines, one circlet on the external border of the peristome, the 

 other about the middle of the body (Collin). 



The pellicle may be greatly thickened to form a lorica, as in Coleps, where it 

 is composed of a series of plates ; or may be decorated with warts or sculpturings 

 of various kinds, -formed as local thickenings, as in some species of Vorticttta, etc. 



The body is often enveloped in a protective sheath or envelope secreted 

 by the animal, especially in sedentary forms. The animal may then be capable 

 of protruding its body from the sheath, and retracting itself back again into 

 it, and when retracted the aperture of the sheairh may be closed by a special 

 lid, or operculum. In the Tintinnidce, some species of which are free-swimming, 

 others sedentary, the body secretes a shell or house, to which foreign bodies 

 may be added, derived for the most part from the faecal pellets of the animal 

 itself ; the structure of these shells has been studied in detail by Entz (53). 



The sedentary habit of life occurs in species of all orders, though especially 

 characteristic of Peritricha. The mechanism of fixation varies greatly in 

 different cases. Stentor attaches itself by cilia, and also by pseudopodial 

 processes thrown out from the point of fixation, and from this type is to be 

 derived that of the Tintinnidce (Faur^-Fremiet, 836). 



The hypotrichous genus Ancystropodium (OxytrichidcE) swims freely or 

 attaches itself by its posterior cirri ; the body us then drawn out at the hinder 

 end into a long stalk (Faure-Fremiet, 837). Trachdius ovum possesses a 

 conspicuous, sucker-like organ by means of which it attaches itself to the 

 stalks of Epislylis-coloniea, in order to devour the members of the colony 

 (Hamburger, 841). 



In the Peritricha the attachment may be permanent or temporary ; in the 

 latter case the animal fixes itself by a sucker-like organ of the aboral pole. 

 In Trichodina the adhesive organ is surrounded by a ring of cilia ; in Cydo- 

 chceta there is an additional circlet of stiff bristles ; in Licnophora the disc of 

 attachment is in the form of a cup surrounded by four concentric ciliary 

 membranes (Stevens, 872) ; these three genera, and others with similar modes 

 of attachment, are ectozoic forms, attaching themselves to the skin of various 

 aquatic animals. Faure-Fremiet (834) has traced the evolution of the per- 

 manently fixed Vorticellid type from temporary fixation by an aboral sucker. 

 As a starting-point is taken Hemispeira asterice, which attaches itself to the 

 gills of Asterias by a bundle of fixative cilia. In Scyphidia fixation is by a cup- 

 like sucker containing a circular brush of rod-like processes, equivalent, 

 apparently, to the fixative cilia of Hemispeira. For this brush-like organ 

 the term "scopula" is proposed ; the cilia in it have lost their motilityand 

 secrete a terminal chitinous knob. Epislylis fixes itself in a similar way by 

 means of a scopula, of which each rod forms a secretion of albuminoid nature 

 (Schroder, 865), which grows continually, forming a bundle of delicate tubes 

 composing the stem, and ensheathed by an outer covering secreted by a 

 rim round the scopula ; the stem that results is a non-contractile structure 

 representing a secretion of the body, and not a prolongation of the body 

 itself. The contractile stalk of Vorticdla, Carchesium, etc., arises by an out 



