Zöology; a textbook for colleges and universities . adapted, as are the flatfishes, for life onsandy bottoms, where they offer little resistance to thecurrents or tidal movements of the water. 6. The Holothuroidea, shaped like a cucumber or aworm, at first present no resemblance to the we imagine an echinoid to be soft, and to be elongatedby pulling at the oral and aboral ends, it will assume aform resembling that of a holothurian. Instead ofhaving a hard shell, the sea cucumbers possess only animperfect skeleton, usually in the form of minute spic-ules, reminding us of thesponges
Zöology; a textbook for colleges and universities . adapted, as are the flatfishes, for life onsandy bottoms, where they offer little resistance to thecurrents or tidal movements of the water. 6. The Holothuroidea, shaped like a cucumber or aworm, at first present no resemblance to the we imagine an echinoid to be soft, and to be elongatedby pulling at the oral and aboral ends, it will assume aform resembling that of a holothurian. Instead ofhaving a hard shell, the sea cucumbers possess only animperfect skeleton, usually in the form of minute spic-ules, reminding us of thesponges. In some casesthe skeletal elements areentirely absent. It is dif-ficult to preserve goodspecimens of holothuri-ans, because of theirbehavior when they turn in-side out, or rather extrudethe internal organs of thebody. The first partsextruded are the Cuvier-ian organs (part of therespiratory apparatus),which form a tangle ofsticky white thread, en-veloping and rendering From Pern*helpless any creature FIG. 53. A sea ECHINODERMA TA 22$ which has had the temerity to attack the large lobster has been seen thus ensnared, and quiteunable to move. It might be supposed that this modeof defense would be fatal to the sea cucumber, butthat animal merely goes into retirement for a time, andregenerates the lost parts. The wormlike species (Synapta) behave posterior part of the body is amputated, while thehead with its feelers buries itself in the sand or Echinodermata, whatever their origin, have them-selves given rise to no other groups. They represent aseparate branch of the tree of life, as do the sponges andmollusks. CHAPTER THIRTY BRYOZOA Characters i. THE Bryozoa (the term meaning moss animals) ofBryozoa n are small aquatic creatures, mostly marine, nearly
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
