INFUSORIES. 39 



structure. Some, in which no oral 1 aperture can be found, must 

 receive nutriment by means of absorption through the external 

 surface alone. Others have a mouth, usually surrounded by cilia, 

 by the motion of which a current is produced in the water, carrying 

 the food which it may contain to the mouth. The subsequent 

 transmission of nutriment throughout the body is effected in spaces 

 of a vesicular form which contract again when their contents have 

 been expelled : they are pushed on by others that have more lately 

 come into view, and have motions that cannot be reconciled with 

 the notion that they possess proper walls. We must rather suppose 

 therefore that these vesicular spaces are excavations in the gelatinous 

 tissue of the body. The undigested portion of the food is, in many 

 of these creatures, cast off by another aperture 2 of the body : in 

 others it escapes through the same opening by which it was received. 



Special organs of Respiration have not been recognised. The 

 external surface of the body appears to be the seat of that function. 

 Still less are there any blood-vessels : perhaps those spaces, which 

 in many species are seen to contract and expand, and which are 

 various in form and number, may serve for moving and transmitting 

 the nutrient fluid which supplies the place of the blood 3 . 



Although no muscular fibres 4 are present, these animals possess 

 the power of motion in a great degree. Some move slowly, others 

 very nimbly. As organs of motion the greater number have cilia. 

 In some are produced by contraction all sorts of changes in the 



1 [STEIN considers all ciliated infusories without a mouth to be larval or embryotic 

 forms of other creatures. Die Infusionsth. a. 181.] 



2 [According to STEIN there is no ready-formed anal opening in any infusory. In 

 those families which do not reject their excrement by the mouth, it is forcibly pressed 

 through the integument of a determinate region of the body, more or less extensive, and 

 not sharply defined. After the exclusion, the rupture is completely closed again. It 

 is allowable, therefore, to speak of an anal region, not of an anal aperture. Die Infu- 

 sionsthiere. p. 24.] 



3 EHRENBERG holds these contractile spaces, or vesicles (?), to be vesiculce seminales. 

 The testiculus is with him a structure whose connexion with these vesicles he has not 

 demonstrated, and which v. SIEBOLD regards as a nucleus, whilst he compares the 

 entire infusory to an organic cell. 



4 In the hollow pedicle of Vorticella, and other similar genera, there is a longitudinal 

 muscle which by its contraction effects the spiral retraction of this part. EHRENB. Die 

 Infusionsth, s. 270. [CzERMACK denies that the contractile streak in the canal of the 

 stem is a muscle : see SIEBOLD and KOLLIKER'S Zeitschrift fur wissenschaftliche Zoolo- 

 gie, iv. 442. Also see STEIN, op. cit. p. 81.] 



