Scrapy找不到蜘蛛

我在scrapy文档中做scrapy教程.这是我当前的目录如下:

.
??? scrapy.cfg
??? tutorial
    ??? __init__.py
    ??? __init__.pyc
    ??? items.py
    ??? pipelines.py
    ??? settings.py
    ??? settings.pyc
    ??? spiders
        ??? __init__.py
        ??? __init__.pyc
        ??? dmoz_spider

dmoz_spider.py与scrapy教程页面中描述的相同.

import scrapy

class DmozSpider(scrapy.Spider):
    name = "dmoz"
    allowed_domains = ["dmoz.org"]
    start_urls = [
        "http://www.dmoz.org/Computers/Programming/Languages/Python/Books/",
        "http://www.dmoz.org/Computers/Programming/Languages/Python/Resources/"
    ]

    def parse(self, response):
        filename = response.url.split("/")[-2] + '.html'
        with open(filename, 'wb') as f:
            f.write(response.body)

然后我从当前目录运行此命令

scrapy crawl dmoz

但我收到错误消息:

2015-12-17 12:23:22 [scrapy] INFO: Scrapy 1.0.3 started (bot: tutorial)
2015-12-17 12:23:22 [scrapy] INFO: Optional features available: ssl, http11
2015-12-17 12:23:22 [scrapy] INFO: Overridden settings: {'NEWSPIDER_MODULE': 'tutorial.spiders', 'SPIDER_MODULES': ['tutorial.spiders'], 'BOT_NAME': 'tutorial'}
    ...
        raise KeyError("Spider not found: {}".format(spider_name))
    KeyError: 'Spider not found: dmoz'

有什么建议我做错了吗？我已经检查了堆栈溢出中的类似问题,并按照那里的解决方案.但我仍然得到错误.