Chap. IX.] NOTOCHORD. 313 



Ceplialocliordata throughout their life, and the 

 Urochordata either permanently, temporarily, or 

 never, have an internal organ of support in the form 

 of a rod, the so-called notoctiord or dorsal rod, 

 which lies just beneath the central nervous system ; 

 the substance of which this rod is formed appears to be 

 allied to cartilage ; in Amphioxus it becomes much 

 more complex in structure than in other Chordata, so 

 that we have here an example of how an organ of an 

 animal which remains at a certain grade in the scale 

 of development, becomes more complex and elaborate 

 under its conditions than does the same organ in a 

 " higher animal," where it serves only a temporary 

 purpose. In the Urochordata the notochord is only 

 found in the tail ; it either is persistent, or is aborted 

 when the free-swimming larva settles down to a fixed 

 mode of life, or it is never developed at all ; as in the 

 rest of the division, it has a stout continuous sheath, 

 and, as it is elastic, it brings the tail back into posi- 

 tion when the organ has been bent by the muscles 

 attached to it. In the Urochordata, therefore, the 

 notochord, when present, may be regarded as having 

 also a locomotor function. 



Amphioxus has no external skeleton, nor have 

 those Urochords that are tailed throughout life j in 

 the rest, the " outer mantle," or test, may become 

 very strong and rigid, so as to form a complete 

 organ of protection; it is remarkable for containing 

 cellulose, a starchy compound which, so common in 

 vegetable organisms, is only known among animals 

 in the Tunicata and the protozoic Cilio-nagellata ; 

 scattered calcareous spicules are not unfrequently 

 deposited in the cells of the mantle, but never form 

 a continuous layer. The tailed forms, such as Ap- 

 pendicularia, are able to rapidly secrete an invest- 

 ment, the so-called " house ; " in this, however, they 

 do not dwell permanently, but are described as 



